Personally I would choose the Gtx 780, you could pair it with a cheaper Motherboard if you want to save a little.
If it's mostly shader or other GPU intensive stuff then the GTX780 would easily win out.
If it's mostly data being pulled from Video RAM then the GTX770 with 4GB of VRAM would pull ahead. (heavily mod'd Skyrim for example)
But in most cases the 780 will easily out preform the 770 4 gb