They are very close generally. Depending on the game, both can edge out the other (usually primarily due to optimizations from partners via "The Way It's Meant To Be Played" (nVidia) or "Gaming Evolved" (AMD)). I'd go with the 7950 myself (assuming same price) even if just for the extra GB of RAM.
^Both those guys are right; the 7950 is slightly better than the 660ti, although it only competes with a 670 if only the 7950 is overclocked. Still, it's a better deal right now than the 660ti, so I'd go with it.