The T2i is a great camera if you can find one on closeout. I actually prefer it to the T3i. There are a few differences, but the big one is that the T3i has a swing-out LCD display, which is useful if you plan to shoot video. It's a bit harder on the battery, though.
There are some significant differences between the T3i and T4i, most notably that the T4i has a newer-generation image processor. It's able to handle focus while filming video a lot better, and is supposedly just generally a better overall image processor. Neither is a "starter" camera, but the T4i is very close to the "prosumer" range that is traditionally a step up from the Rebels.
Nikon's DSLRs are also very good.
In general, though, I agree with photo_chick. Figure out your budget, figure out the features you're looking for, then go to a store and try each of them in your hands. The main reason you keep seeing Canon and Nikon mentioned is that they're the biggest players, not that they're necessarily the best players. On the contrary, I'd argue that at the consumer level, Canon, Nikon, Sony and Pentax are all very, very close in terms of image quality. Nikon and Canon will give you the broadest range of lenses and accessories, but most people won't have more than a small handful of lenses, and all the brands cover all the main bases.
Put another way, at this level, the quality of your photo won't be determined by whether you have a Canon or Nikon: it will be determined by your skill as a photographer. Equipment comes more into play when you're getting into heavier-duty photography and can afford to drop $1000+ on a lens. Once you're at the point where you're taking excellent photos with a kit lens or inexpensive fixed lens (such as the Canon Nifty 50), you may want to begin exploring what high-end glass can do for you. But that really won't matter until you've mastered your eye.