I've got at least in-store experience with both. Both use the latest in Canon's excellent sensor, image processor, and UI/control technology...you really can't go wrong with either.
That said, some key differences:
- XTi is smaller; some love this, others hate it. Question of personal ergonomics.
- The viewfinder on the 30D is larger than that of the XTi - feels brighter, too, regardless of lens.
- The 30D has better image quality and far better low-light/high-ISO performance, very close to the Canon 5D in noise control at high ISO's
- The 30D's construction is noticably chunkier and robust (that said, I've had a Digital Rebel, the original 300D, for four years and use in sand, dust, dirt and even a flood have proven even that old model to be pretty robust).
- Shutter lag seems less on the 30D, and CF writes seem faster. This is perhaps subjective..? (Also subjectively speaking, I find 30D operation to be faster, with two dials you can use simultaneously - not an issue if you don't shoot Manual all the time)
- The XTi is lighter, as befits this forum. :-D
I tested them all, and as a digital imaging professional, I took the weight penalty for image quality and got the 5D. The 30D really is like a mini 5D, having more in common with that camera than the XTi.
Be sure to check out dpreview.com for massively in-depth reviews...and, really, you can't go wrong with either camera. Just depends on what tradeoffs you can stomach and kinds of images you make.