It depends who you are, this comparison is more about video than pictures, but if you are photographer first that uses a lot of remote flash than I would go with the Canon T3i 600D because of the built in flash transmitter.
OK now that we have eliminated the photographers for you people that shoot video the decision becomes a little harder.
Very interesting; watch the comparison at the beginning at full screen and then look at the aliasing on my sweatshirt on the left (T3i), why is the T3i worse?
First off the sensor is exactly the same, I have been testing this for a month now and I have not seen any difference at all in image quality.
Depending on where and when you look there is $100 to $200 difference between the two, with the T3i being more expensive.
The first major difference is the flip out screen. At first I was like oh I might break it, but I have come to really love it. Low shots are great, so much easier when using a slider on the ground. If you like to wear it around your neck and have zipper on your jacket it can protect the screen from getting scratched! The specs on the T3i and T2i for the LCD monitor appear to be almost identical except for the angle of view which the T3i has 10% bigger than the T2i, for the past month I have noticed that the T3i screen looks slightly better.
If you are not a hacker, meaning you are not comfortable with items like upgrading your camera’s firmware then the T3i is for you because of the manual audio level controls, if you are comfortable with this hack and using beta software that will sometimes ruin a shot because Magic Lantern just stops recording the the T2i might be for you.
Manual zoom is kind of cool, but to be honest I would rather use the right lens for the job instead of a quality hit.
Snap shot video which is new on the T3i, I am a little baffled by this half baked idea Canon came up with, it is like they are trying to compete with imovie on the iphone buy assembling small clips and editing them in camera, that is kind of cool but it is not even close to the iphone interface and you can upload it from your camera so to me it is a worthless feature.