If having to choose between these two, definitely this in the terms of performance - it 'd give you >10 % more FPS, regardless of the overclocks there.
But you have to be careful with the PSU. GTX 670 needs 2x6 pin (170W), while 770 8+6 (230 W). Also both these units are factory-overclocked, so expect some more TDP, like 180-186 W and 235-244 W respectively.
Anyway, if there is a cheap 950/960 or 1050 as if price is close, you can get one of them instead, as they might get a bit better performance with the better-touched drivers, while also wasting less watts and running with lower temperatures.
In my country it's a bit of amazing how often one can see a cheap 1060 3 GB, while it's a GPU that covers better the minimum game settings in 1080p.