AMD GPU are more powerful than NVIDIA ones but I pick NVIDIA over AMD because of driver stability problems on AMD.
AMD is more powerful at Direct Compute
Nvidia cards are faster depending on the Software
Most games and applications will be suited for either or and depending on what you like to play and if PhysX is something you really care about and also PhysX can be used by your CPU do not have to depend on a Nvidia GPU for that
Also take into fact that AMD has released Mantle which will help lower end cards perform better bound by CPU Bottleneck this will manipulate the GPU more and take less load off the CPU
The reason this has come about because AMD cpu are not superior to Intel and this will make up the lack of CPU power pretty much
Factors for a better GPU experience will be CPU /Overclocking and PSU these three things will ensure top tier performance you also don't have to pair Brand with brand they all work great together
Now the most important this is how much your willing to spend and how much performance are you looking for
for right now Nvidia cards would be GTX760 -Mars GTX770-SC GTX780-TI
AMD R9-270 R9-270X R9-280X R9-290 R9-290X
For the sake of humility the AMD cards perform best under water 290 /290x
but prices match NVIDA's top tier cards with better cooling that is what you will have to look at you want a HOT card you have to buy extra cooling for or a card that is respectable without having to go all out over ?