The Canon Rebel T3i takes the consumer level dSLR a couple steps closer to the mid-level Canon 60D with the addition of the rotating rear LCD screen, remote flash firing, and in-camera processing features. The already highly competent, older Rebel T2i already shared many important features with the 60D (and even features of the semi-pro 7D) including the 18 MP sensor, 63-zone exposure metering system, high ISO performance, HD movie capabilities, and Digic 4 image processor. With these new upgrades, it might make it even more difficult to choose between them. But there are some important differences.
If you are considering the Rebel T3i vs T2i, the Rebel T3i is replacing the T2i. Since both cameras share the same 18 megapixel sensor and Digic 4 processor, both the T2i and T3i will create images with exactly the same image quality, produce the same low light/ high ISO performance, shoot at 3.7 frames per second, and have nearly the same size and build quality. They are both offered with the same 18-55mm kit lens (with some minor cosmetic differences on the new T3i kit lens). The T3i is very slightly larger and heavier due to the addition of the rotating rear LCD monitor. And that is one of the biggest differences between the two cameras. Do you want and need a vari-angle rear screen or not? The other major difference is the ability of the T3i to remotely control multiple off-camera flashes. Like the 60D and 7D, you can use the built-in flash of the T3i to trigger other Canon Speedlites. Some other minor additions to the T3i include the Scene Intelligent Auto Mode, which is a feature borrowed from point and shoot cameras. When in Auto mode, the T3i will make a determination of what type of scene you are shooting - close-up, portrait, landscape, etc. - and automatically configure the camera settings accordingly. However, if you want to use a powerful and costly digital SLR as a point and shoot, you should probably save the money and just buy a nice, high quality point and shoot like the Canon S95. Other additional but not essential upgrades include the in-camera processing Creative Filters, and the ability to choose different image size ratios and to rate your images. (Helpful hint: press the Q Button while in image playback and you can access features like rating, rotating, and Creative Filters.) There is also a marginally helpful Feature Guide which gives brief descriptions of various settings and some additional video features like Video Snapshot, which you can use to shoot short video clips that are automatically joined together into a video, with music.
Canon Rebel T3i vs. 60D vs. 7D Sensor and Image Quality: All three cameras share a very similar sensor and 18 megapixels, and so their image quality will be virtually the same. All are capable of taking professional quality images.
Exposure Metering: The three cameras all share the latest 63-zone, dual-layer exposure metering system and 4 metering modes. That means they will all determine the exposure virtually identically and enable you to take properly exposed photos in most every situation, including difficult back-lit scenes. The size of the areas metered for Partial and Spot metering vary slightly between the cameras, but that isn't anything critical.
Autofocus: The T3i shares a similar autofocus system to the 60D, with 9 focus points and three auto focusing modes. However the 9 AF points of the 60D are more sensitive than those of the T3i: all are cross-type in the 60D, only the center is cross-type in the T3i. The 60D autofocus system is much less complex than the sophisticated AF system of the 7D with its 19 AF point system and its additional Zone, Spot, and Expansion focus modes. These various modes address how you want to deal with and group the numerous AF points. Plus the custom settings of the 7D allow one to customize how the AF system works - how it tracks subjects, how it deals with objects that come between you and your initial subject, how quickly it responds to these changes of possible subjects that are at different distances from you, etc. However, if you are not an avid sports photographer, a wildlife shooter, or someone who understands, needs, and will use the elaborate features of the 7D AF system, then this shouldn't sway you.
Construction: As you can probably figure out from the prices, each camera is not built the same. The T3i has relatively strong construction of a stainless steel frame with polycarbonate body. The 60D has a stronger and lighter aluminum frame and polycarbonate body, but not as strong as the 7D's magnesium alloy construction. The 60D also has some amount of weather sealing - more than the T3i, less than the 7D. But for most users, including even those using the camera daily or in travel situations, the construction of any of these cameras is far more than good enough, strong enough, and durable enough.
ISO: Since they all share a very similar sensor, the ISO sensitivity and performance at high ISO settings is virtually the same for these three cameras. But don't take my word for it, don't be swayed by pixel peepers on forums, instead check out the camera sensor tests at dxomark to verify this. As you can see, they all share the exact same overall score, and show very similar performance.
Controls: As with construction, the buttons and controls vary with these cameras. Unlike the T3i, the 60D and 7D have nearly every control an advanced photographer needs on the exterior of the camera and they also have the top LCD panel and rear Quick Control Dial that are not on the T3i. With all the cameras, any controls can also be easily accessed with the Q Button and Q Menu or in the other menus on the rear LCD monitor. The top buttons of the 60D set only one setting each, so this is less complicated than the multiple-setting buttons of the 7D. Canon has removed the white balance (WB) button on the 60D that the 7D has, but that isn't a big deal - use the Q Menu. Another change on the 60D is that the Multi-controller has been moved from the thumb joystick like the 7D and 50D and placed in the middle of the rear Quick-control dial. This doesn't change how it functions, and should just be a matter of getting used to the difference. If you plan on using your camera on Auto or Program most of the time, then the controls of the T3i are more than sufficient for your needs. If you work in Av, Tv, or M modes and need quicker and more direct access to your controls and the additional top LCD screen to view and change your current settings, then you need to consider the 60D or 7D over the T3i.
Menus and Custom Functions: These allow for greater control over customizing how the camera functions. The T3i has less Menu and Custom Function setting options than the 60D, and the 7D has yet a few more than the 60D. These settings enable you to customize the operation, function, and controls to work how you want them to, including things like exposure increments, peripheral illuminations correction for lenses (fixes dark corners), tweaking how the autofocus system operates, setting more precise white balance settings, and customizing which button does what. There are ebooks such as my Canon T3i Experience - The Still Photographer's Guide to Operation and Image Creation With the Canon Rebel T3i / EOS 600D and Your World 60D - The Photographer's Guide to Operation and Image Creation with the Canon 60D which walk you through all of the Menu settings and Custom Function settings so that you can set up your camera to work best for how you photograph, and also begin to learn to master all the advanced features, settings, and controls of these powerful dSLR camera.
Wireless Flash: Like the 7D and 60D, the T3i incorporates wireless flash triggering. This allows you to trigger multiple off camera flashes at different output levels. The T2i does not have this feature.
Articulating LCD Screen: The big new feature that the 60D and T3i have that the 7D and T2i do not is the articulating rear LCD screen. This may prove useful for videographers, as well as for setting up compositions while the camera is on a tripod, for macro use, or for using it from unusually low or high vantage points. Some users will be able to avoid buying an expensive angle finder because of this feature. There is also an electronic level in the 7D and 60D, visible in the viewfinder, rear LCD, or top LCD.
Viewfinder: The T3i has a pentamirror viewfinder with 95% coverage of the actual resulting image. The 60D has a large, bright pentaprism viewfinder with 96% coverage, not quite as nice as the nearly 100% view of the 7D pentaprism.
Processor: The T3i shares the same Digic 4 processor as the 60D. The 7D has dual Digic 4 processors. However, if you don't need to shoot dozens of continuous images, you probably won't notice any processing speed issues.
Continuous Shooting Speed: The T3i can shoot 3.7 frames per second. The 7D can shoot a blazing 8 frames per second, in which the photos barely change from frame to frame. The 60D can shoot a respectable 5.3 fps which is actually a more useful rate. If you need the extremely high fps for sports, wildlife, or other action shooting, get the 7D. If not, don't be swayed by this excessive feature.
Memory Card: The T3i and 60D use the SD memory card. The 7D uses the CF card.
Battery: The T3i and T2i use the smaller LP-E8 battery with less capacity than the LP-E6 battery used by the 60D and 7D.
Size and Weight: The T3i is smaller and lighter than the 60D, which in turn is smaller and lighter than the 7D. Go to a store and hold them to get a better feel for their size, weight, and feel. The 60D and 7D "feel" like the more substantial cameras that they are. A nice improvement of the T3i is that its hand grip area has been modified, and has a different feel than that of the T2i - the area where the thumb rests is contoured differently and has a nice channel for the thumb, which allows for a much more secure one-hand-grip of the camera.
AF Microadjustment: The 7D has this feature, the 60D and T3i and T2i do not. This allows you to adjust the focus of each of your lenses in case any of them are slightly front-focusing or back-focusing.
Locking Mode Dial: This is a new feature for a Canon dSLR, only on the 60D, that keeps the Mode dial from accidentally rotating. A nice touch.
Full HD video: Of course they all offer this capability. Note that this is not video for your kids' parties and soccer games. It does not have continuous autofocus while shooting, as a camcorder does. It is not designed for that kind of use, but rather for serious videographers who typically manually focus. You can adjust autofocus while shooting by pressing the shutter button or the AF button, but it may have a less than desired looking result and unless you are using an external microphone, the autofocusing sound will be picked up. The T3i has the digital zoom feature in video, which allows for nice smooth zooms while filming.
Flash Sync: the 60D and T3i do not have a PC sync flash socket to plug in PC sync cords for off camera flash use. The 7D has this. However, they all offer wireless remote flash capability with the built in flash as a commander.