T3i user here. Difference between T2i/T3i is negligible. T3i has a swivel screen along with some other minor differences so they're basically the same thing. Personally, I love the swivel screen as it allows you to take some awkward shots without looking awkward yourself. T3i/T4i, the differences are slight but not by much. T4i has a better processor and does video a lot better than the T3i does so that's a bonus if you're into shooting video.
Since this is your first camera, I'd recommend the T3i as it, as well as the T2i, have dropped in price since the T4i came out. You can find pretty good deals on eBay or 1saleaday.com for the cameras. 1saleaday.com routinely has camera sales so if you sign up for their newsletter, you're sure to find a good deal on a camera. I'm not familiar with Nikons at all however, whichever brand you go for, can't go wrong really as both are quality cameras.
Also, you might wanna check out the mirrorless cameras like the Nikon 1 or Sony Nex5/7 as well. While they're not DSLR cameras, they still provide excellent images in a unit that's not quite as bulky as your normal DSLR. They're a smidge pricier then the T series cameras but that's your call.
Overall, I'd say find a nice deal on the T3i with, hopefully, the 18-135 kit lens and you should be set for quite some time.