I have 2 similar rigs here. One is running a 6970, the other a GTX 660 Ti. Both with an overclocked i2500k and 8gigs of 2133mhz ram. Fps is mostly at 60(vsync) with drops to around 45.
I feel no difference at all while playing between the two.
Both cards are watercooled so temperatures are not really a problem, but the AMD card is running about 10 degrees hotter under load usually.
I never had any driver issues with the AMD nor did I have any with the nvidia, both are running dual screen setups and were a breeze to set up.
What performance you'll get in other games is something you'll need to ask someone else.