GTX 770 is somewhat faster than GTX 960 but some games like Witcher 3 are known to favor maxwell architecture and hence perform better in 960. But since GTX 960 has only 2 GB VRAM, I would suggest you to go with GTX 770.
GTX 770 is somewhat faster than GTX 960 but some games like Witcher 3 are known to favor maxwell architecture and hence perform better in 960. But since GTX 960 has only 2 GB VRAM, I would suggest you to go with GTX 770.