Given you are wanting to go triple monitors I would say the GTX 780 with 6GB RAM.
Another post just made me think about this, I monitor vRAM usage in games constantly. With Bioshock Infinite, Tomb Raider, Shogun II Total War, and other games I play, the highest vRAM usage I have ever had is 1800MB with max settings at 1080p.
Triple monitors will cut into this, and probably by next year games will need 3GB or more vRAM, and the triple monitors increases this all the more. I think you are best to get the vRAM. The lack of vRAM in those later games could create a bottleneck before you manage to max the 780ti out.