Was just shooting this past weekend with my friend Dana in MA. He has a D5100. Pictured below is his D5100 on the left and my 7D on the right. The image was shot with my T1i, which is the same size as the T3i (more or less). Dana bought his D5100 with Nikon's excellent 18-200mm lens. He gets some great shots with it and loves the versatility. Having seen my images and his, I'd have to say he does much better in low light (high ISO) shots with his D5100 than I get with my T1i. My 7D is about even with his D5100 in that regard, but it was a damn sight more expensive.
So like the others have been saying, go with whichever one feels better. Just be aware of the commitment as far as lenses go. Once you get a few in your kit, it's hard to change brands.
