Well, if you have to ask, you probably don't need any of the features that only one of the cameras has, and there are a lot of similarities between the two.
Really, it boils down to the feel of the camera. The XTi is smaller and more plastic-y than the 30D, which is heavier. You'll probably want to try holding both to see how they feel; I find the XTi is too small for me to comfortably hold, while the 30D feels just right. Also, check out the way you change settings (shutter speed, aperture, ISO, perhaps white balance (and any other things you might want to change, but those are the most common)) and evaluate that in the context of how you're going to use the camera. You might find you prefer one way of doing it to another (e.g., the XTi shows settings on the main LCD; the 30D has a rear control wheel for changing some settings) or that you can do it faster with one than with the other. If you expect that you'll be shooting in a situation where you'll need to quickly change settings (rapidly changing between different subjects, or in rapidly changing light like at sunset) put some real weight in the one that's faster for you.
Basically, they're both good cameras, and both will take good pictures. Try to find the one that's least frustrating to use, whether it be in size, interface, or features.
Phil Gengler
Overheard in NY Penn: "All aboard! Get on the train if you're coming with us!"