Keep in mind if you are buying it for your tank you MIGHT need a better macro lens then what comes with it...
im shopping for one also.
basically everything Thirty mentioned is true. i just want to add that the XTI comes with a larger LCD screen. IMO, that alone is worth the extra $$$. If u could go look at it physically, i think you'll agree. The lcd is waaaay more usable on the XTI.
If you get the kit, it does not include a macro lens, that will have to be purchased seperately.