I just can't get over that they use "low light" and "Rebel" in the same sentence. Unless the AF is 10000% better than what it was with the XSi, the low light sucks.
Are you going to use video? If not, I would get the 50D, just because it is cheaper, and also the sensor in the 7D/T2i REQUIRES you use top quality glass to utilize the sensor. It is in the review that Phil Askey put out (DPREVIEW). Just putting that out there so no one tries to say I pulled that out of my ace.
Personally, I would wait. New Rebels and such are going to be announced soon IF anything, buy used. That way you can save yourself 30% at least. I would rather have a 5D over both those, as a low click used one is about the same price as a new T2i kit and 50D body.