I have tried both of these, and have come to the conclusion that the DC33 is superior. Here's why:
-The suction and brush bar are clearly more powerful. This is what sold me. I can see the difference in carpet lines (medium pile carpet) when I use one or the other. -35ft cord is a significant benefit over the 25ft cord. I can almost do the entire floor by just plugging in a single outlet. Two would cover the whole house. With the DC25, I had to constantly unplug and replug. -Larger dustbin means I can keep vacuuming for longer without emptying. Also, the design on this one seems to keep the shroud (the perforated shield inside the canister) freer of stuck-on dust. -Quieter -Although a bit larger, it is a similar weight to the DC25. -Critical sections can be easily taken apart to clear blockages. -The DC25 has a tiny plastic piece that keeps the vacuum from tipping over in upright mode due to the ball mechanism. If I tilt the vacuum to the side while it is close to being upright, I can hear a loud snap as the plastic piece is forced into a position where it shouldn't be. I can see this breaking really soon.
There are some shortcomings:
-The wand is a clear cost cutting measure. It is plastic rather than aluminum. Also, the handle is missing (present in the DC25) a contoured plastic piece that makes it much more ergonomic to hold when using the wand. It also requires an extra button to release AND another button to retract the wand. Lastly, when you release the wand from the hose, there is just a bare hose, rather than a similar cuff as the one at the end of the wand with a release button as on the DC25. It is still usable, but not as nice. It just adds to the already cumbersome wand system on the Dysons. -The DC33 switches suction to the wand when in upright position whereas the DC25 switches it when the wand is drawn. A minor annoyance, especially if you're wearing a long sleeved shirt; it may get sucked in when you hold the handle. -No ball, but to me the difference in maneuverability is negligible.
Update 3/4/2011: I have used this vacuum for a few months now. I returned the DC25 and do not miss it at all. The DC33 continues to perform admirably. True to Dyson's claim, the suction is as good as new. I've washed the filters once and the process was simple, much simpler than going to the store and buying filter bags. Vacuuming is a pleasure, due to getting the instant gratification of seeing the the dust collect in the bin, and the clean exhaust. It's probably subconscious, but as a sufferer of seasonal allergies and constant nasal congestion, I feel like I can breathe better when I vacuum.
The only pain has been removing hair from the brush bar. It doesn't pop off, unlike the DC25, so removing hair can be a cumbersome process.
Update 3/18/2016: This thing just won't quit. I keep seeing shiny new Dysons out there and feel pangs of upgrade-itis, but this vacuum just won't give me an excuse. The suction is still great. I'm still using all the original parts, including the original filters, and I don't even wash them as much as I should.