Definitely the GTX660 Ti. This is well worth a look:http://techreport.com/review/23981/radeon-hd-7950-vs-geforce-gtx-660-ti-revisited
I don't think there's anything wrong with the 7950 inconsistency though - that's been observed in both nVidia and AMD hardware since TR started measuring this in 2011.
So 7950 won't look any less smooth than we're used to (since performance fluctuations have been standard until recently). The GTX660 Ti infact was just as inconsistent in the past, which makes me think what we're seeing there is a driver-based solution, because now it's flawlessly consistent.
I think what it boils down to is that at 30fps, a 7950 will look like 30fps. But with a GTX660 Ti, it will look smoother (since we're used to 30fps with a lot of fluctuation, not pure consistent 30fps).
Most people respond to the performance consistency thing by focusing on the inconsistency of the Radeon (which is understandable when that's exactly what the author does). Author should know better though - there's nothing unusual about fluctuations. What's really significant is the lack of fluctuation with the GTX660 Ti, which is a major improvement.
Some people will bring up the overvoltage drama with the MSI PE cards, but it was blown way out of proportion. You could buy a non-PE model if you're really worried about it, but I'd take the PE. Not like it doesn't come with a warranty :-)