I literally just bought the Nikon D7000 on Tuesday. Here is what I learned from reading multiple in-depth comparisons of Nikon and Canon.
On Nikon D5100 vs. Canon Rebel T3i: Overall the Nikon will be a little bit better camera at the moment. Their technology is just a little bit ahead of Canon. In-camera image correction, lower noise at high ISO, in camera HDR: the full list is in that link. The drawbacks to the Nikon are that this model has NO internal motor...so only the AF-S lenses will work with this camera (because they have motors in the lens). Also, one major bonus to the Canon is the ability to manually focus when taking HD video, so if you're wanting more control doing so, that's a plus.
Overall I thought Nikon was better right now, but I didn't want to replace all my lenses, so I went with the D7000 which does have a motor inside.
Anyway, hope that link helps. Also, if you're wanting a cheaper alternative, I'm selling my Nikon D50 + the 18-55 kit lens which are both still in great condition
