Canon Rebel T2i, or Nikon D3100?
Which one?
Why is this question constantly circulating - can't make up your mind?
I recommend what I know and own - Canon
Both are excellent entry-level DSLR's. I'm sure supporters of either system will shout out their names, but the true test is how compatible the camera is with you.
Go to a camera shop and handle both. See how it fits in your hands and how easy or hard the controls and menu systems are for you. Find the camera that is compatible with you, and don't plan on changing yourself to fit a difficult camera.
It's similar to shopping for cars. We could all give you specs or rave about our favorites, but you have to take the test drive.
Remember, a good camera doesn't get in the way of your creative vision.
Nikon d3100
Both are excellent cameras.
However I prefer the D3100. Thinking about it the T2i is $849 while the D3100 is $699, this is however, without tax included. A difference of $200 dollars while the D3100 practically the same functions and set up as the T2i but with less mega-pixels. Kit lens are decent (i think?) But for $949 you can get the D3100 with the kit lens and a 55-200mm NIKKOR VR Lenses. It's only $100 more but you get even better lens for almost the same price as the T2i with kit lens.