If you really need the very high end, NVidia is the right choice, but it comes at a high price. On many of the lower market ranges, AMD often offers the best value (this can also depend on each store offers, not just on list price).
If you need to do professional things, follow @CGI-Quality advice, unless you know for sure you're going to use SW that explicitly prefers AMD (it could become less uncommon in future if devs will like the latest AMD tech enough)

Stwike him, Centuwion. Stwike him vewy wuffly! (Pontius Pilate, "Life of Brian")
A fart without stink is like a sky without stars.
TGS, Third Grade Shooter: brand new genre invented by Kevin Butler exclusively for Natal WiiToo Kinect. PEW! PEW-PEW-PEW!