The Nvidia GTX260 216 is slightly more powerfull, runs cooler, uses less watts at idle but a little more at peak. In benchmarks it's neck and neck, with either card taking the lead depending on the game and resolution. The GTX is generally faster, while the Radeon runs some games better and typically performs better at very high resolutions, thanks to a bit of extra memory.
http://techgage.com/article/ati_hd_4...mb_follow-up/1
IMHO, between those 2 cards it comes down to preference.