I'd go for the two GTX 780's. Way more bang for your buck, and 780 prices have really come down to the point where they should be the default choice in the $400-$500 range in my opinion.
The Choice you need to make is how much money do you want to spend if you want longer living cards get the two GTX 780's, Less stress on two cards than one, if you want to go the cheaper route go with GTX 780 ti it can push 3 1080p monitors but with a little more wear and tear over the life of the card!!!