One big difference is the D3000 doesn't shoot video. Is it worth paying $150 more to be able to shoot video? Only you can answer that. Also, look at the megapixels. The D3000 is 10.2 megapixels, whereas the D3100 is 14.2. Do you plan to print posters from your shots? If all you're ever going to need is 4x6s and maybe the occasional 8.5x11 then you'll never miss the extra pixels. In fact, unless you tell it to shoot smaller pictures, you'll actually eat up space on your memory card faster with the D3100.
If you aren't advanced enough to know about the other differences between the two, I'd save money and go with the D3000. If you really want to be able to shoot video with your camera, then the D3100 is a no-brainer.
True, I believe that a CMOS sensor is better than a CCD sensor, however, in practice, I doubt it'll make that much of a difference when you consider the lenses that you're using, the subject matter, person shooting the picture, etc. I'm sure you'll be more than pleased with the shots you'll get with the D3000, and probably won't say "this picture would have been way better with a CMOS sensor."
Besides a few features, and the lack of a internal motor for focusing AF lenses, the D3000 is superior in every way. The D3000, though it's supposed to replace the D40 it's two steps above it and about half a step above a D80 when it comes to image quality.
See the DxO comparison... http://www.flickr.com/photos/bizzyb0t/4711236246/
No, the D3000 does not have Live View, the ability to use the LCD as a viewfinder. Live View isn't as useful on a dSLR as you might think, though -- it doesn't work as smoothly as with a point and shoot camera and it uses an inferior slower auto-focus method. In nearly all cases, you would want to use the optical viewfinder on a dSLR for the highest possible auto-focus performance anyway otherwise you lose one of the advantages of having a dSLR.