Dont go with GTX 660, if you have the budget to buy one, then invest it on a 690, its better to add a couple of dollars to GPU, dont go so cheap with it.
OT: I would pick Nvidia because of its Physics and dimensionals.
What's your budget for your new GPU?
also download Speccy from Piriform and screenshot it so I can see your full system spec. Also what wattage is your power supply?
Hmm im not the best with this but ive heard of AMD and i havent heard of NVIDIA so id have to say AMD. Best of luck finding the best one. Try searching up on youtube reviews of them.
How can you hate them when a £320 GPU from AMD will perform the same or better than a £450-500 GPU from nVidia?
Also Mantle is a new API that AMD are implicating (and allowing nVidia to use it) for console and PC game developers to use to make games much more optimised for the hardware of everyone's machine, whether it be console or PC, to give the best gaming performance and experience possible. This'll be put into place in the next month or so and it'll change things MASSIVELY.