The 7870 performs better in most games, and amd's drivers are not as bad as people would have you believe. They are actually pretty good. However the 660 uses less power so if you plan on doing sli or crossfire or you have a lower wattage psu you may want to choose it.
AMD is actually doing better across the board. The 7870 is ahead here or 7870 LE is even better. Plus you get bioshock infinity and tomb raider free going AMD