Here's a comparison on RIM's site:
BlackBerry - Compare BlackBerry Smartphones and PDAs at BlackBerry.com
8520 has no 3G (I told you you'd get 3G in a coverage are in another discussion. My bad - the 8520 has no 3G) and no built-in GPS. On the plus side, the 8520 has double the internal memory of the 9000 and has the trackpad instead of the trackball.