What's the rest of your setup? And it depends on what you're doing with them.
960 is roughly $200
whereas
770 is $300-$400
770 may be 10-15% better in overall performance but the 960 is more than a year newer (better driver support) and the 960 only needs 120W where the 770 needs 230W, so your PSU is significant.
I'd say go with the 960 considering it's newer, cheaper, requires less power, and is only slightly lesser in terms of gaming performance, especially compared to the price.
Here is the site I pulled this info and there are plenty of more educated people on this forum so I'd wait on their take but here's my 2 cents anyways
http://www.game-debate.com/gpu/index.php?gid=2436&gid2=881&compare=geforce-gtx-960-2gb-vs-geforce-gtx-770