The 7970 is indeed a bit faster, and if you can get a non reference version, do so.
When BOTH overclocked you can expect pretty much identical performance from the two cards and the only real deciding factor is that extra 1gb of vram on the 770, and preference on brand.
You will get lots of fanboy hate over this specific issue, as they were both essentially the flagship single gpu cards of last generation
If you are only playing at 1080, I would suggest the 7970. If you ever plan on doing 3 monitors, or upgrading to a 1440p, then i would suggest the 770.
FINAL deciding factor right now is the game bundles. I'm not sure if the 7970 still comes with the plethera of free games, but i do know that the 770 comes with batman arkham origins, ac4 black flag, and splinter cell, as well as $100 off sheild.
Basically if they both have game bundles still, go for whichever has the games you prefer more