Some users say drivers, but AMD drivers are honestly pretty good and improve and optimist game performance with each release.
For me personally.
AMD
+Great game bundles
+Performance per dollar
+More video options(for watching DVD/BD/ect)
+IMO great driver improvements over time(game performance increases. Nvidia has similar, but i think AMD may have an edge at times.)
-Noise(can be overcome with 3rd party cards)
-Not quite as advanced control panel(but they have improved the per game profiles ALLOT with the newer drivers)
Nvidia
+Much more quiet at the higher end
+Nvidia inspector(third party program, but very useful. Radeon Pro has a long way to go to compete with this.)
+Lower power consumption at peak loads(680 VS 7970 @ furmarks loads).
+PhysX for the games that use it.
-Poor game bundle(pay to win game money. Not ideal in my opinion)
-Not as many video filtering options(DVD/BD/ECT)
Overall, it is hard to call it anything but a tie.
Personal preference may lead a user one way or the other. I would like a quieter card(as well as a blower style cooler, but AMD's current blower style cooler is WAY too loud for my taste.), but AMD's game bundle is FAR better IMO.