2 770s.... I currently have 2x EVGA GTX 770 SC ACX in SLI and it out performs 1 780ti across the board.
HOWEVER..... if you are only running 1 monitor (1920 x 1080) it maybe worth getting the 780ti now and adding another later down the road when you feel like you need the extra performance.
I have 4 monitors and run in 5760 x 1080 in a lot of games which is why I chose 2 x 770s because I wanted the added VRam