Pro's for the XTi: 10 mp and sensor self cleaning.
Cons: ISO range and performance isn't quite as good and at least for my XTi metering seems to be off as well.
30D pros: more substantial body, better AF
30D cons: price, 8 mp, have to clean sensor yourself.
If you're going to keep the body for a while I'd go with the 30D, you'll appreciate it's features more if you really get into it. If you want to just get started the XTi is a good little body, plus I think we'll see the next generation of the 10/20/30D series this fall.
I'd start with the Rebel and by then you'll know what you need/want and have more choices.