The GTX 770 gets 42 fps minimum and 54 fps average. That's pretty good IMO.
The GTX 780 gets 49 fps minimum and 65 fps average (but your monitor is probably able to refresh only 60 times per second...)
I think you'd be happy either way. Get the GTX 780 if you're planning to add a second monitor, or to upgrade to 2560x1440. Otherwise, the extra $170 is hard to justify.