The title goes back and forth, but for now Radeon is the performance leader according to several PC sites, including PC World. nVidia is really good. So, you can't go wrong with either if you can afford one of their top end offerings. It doesn't have to be the high priced latest, but one of the more recent top end versions. Sammorriss seems to have shown you some good offerings from both.
Until nVidia drop their prices, however, ATi cards seem a lot more competitive, an X1900XT can be had for under $300, a rival 7900GTX will set you back $450.