Basically, the T1i is a year newer, and has a slightly higher MP sensor, and video. You probably should go for the XSi and spend the extra cash on a good lens. For convention use, the cheapest best lens would be a 50mm f/1.8. Also a good choice would be the 35mm f/1.8.
Hi, I work at a used electronics store, and I am able to get expensive electronics for super cheap. For the past few years I have been going to cons (anime and comics) using a compact camera and a htc evo (8mp) to take pictures. Of course they're all bad, and it takes forever for them to take the pic.

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