Hello,
You really can't go wrong with either option. As you have probably noticed from playing with the cameras at the store, the 30D has a very different user interface than the XTi. Most pros/enthusiasts find having two wheels for adjusting settings to be faster than using buttons and menu options, although the on-screen menus can be more helpful for someone just starting out.
I don't think you'll notice the differenct between 8 and 10 mp, but realize that most folks change digital camera bodies once every two years or so. Therefore, it makes more sense to invest carefully in good lenses and purchase the least expensive body that will meet your needs at the moment.
That said, these two bodies are pretty close in price after Canon rebates. I would recommend the 30D and the EF 50mm f/1.4 USM lens over the XTi with the EF-S 17-85mm f/4-5.6 IS USM lens. After Canon rebates, you can get the 30D/50mm combo for $1,224.90 v.s. $1,209.90 for the XTi/17-85 combo.
You'll give up the versitility of having a zoom lens by going with the 30D combo, but most folks here would agree that a fast 50mm is a better lens to learn with. You'll also have a camera that you won't out-grow as quickly, and you won't have to worry about selling your EF-S lens to fund the purchase of a better normal zoom like the EF 24-70mm f/2.8L USM or EF 24-105mm f/4L IS USM. You'll always have a use for the fast 50mm.
Here's a link to B&H part numbers that I have typed up that you may find handy as a reference:
http://dsnyder.ws-e.com/canon_2006/