I've been more on the side of ATi through the years until 2007 when I decided to go with Nvidia (8800 GTS 320MB). That card is still amazing. When I got my new Asus I decided to switch back to ATi since the Radeon HD 5870 was too tempting of an offer.
My old Radeon X1650 is still running in my old Shuttle XPC, while it's outdated for games it's amazing for all other media e.g. video rendering, capture, encoding/decoding, visual plugins in media players, all the basic stuff and, of course, XBMC as my media management suite.