There is no comparison between them, I mean they are in such different price points and performance realms. The G12 in a Canon case with strobe will cost say $1000, but a D3100 system will cost probably $5000. I'm sure you know that, but to me they are hard to compare.
If you want good video, neither of them, in fact no still camera, does as well as a dedicated video camera. But for that you need a very different lighting setup, and editing the video is a huge chore. So decide what mix of still and video you like, before you leap.
If your primary interest is still photos with an occasional video, I'd just live with what you have until 1080 is more common in that class of camera. I think Panasonic and Sony still cameras do a better job than Canon right now. Panasonic and Sony seem to bring their camcorder technology to the still cameras sooner. But I have not seen anything to rival a true video cam yet.
If you do want to go to the D3100, good for you. I think it offers a lot of bang for the buck. It's the number one seller among DSLR cameras at Amazon right now, so people seem to agree. It does do 1920 by 1180 at 24fps into .mov format, not much compression, so run time is limited compared to a camera that does AVCHD. Those brief videos would be impressive on youtube, but not for a full length movie.
Why not ask some questions in the video forum too??