Neither are particularly bang for your buck, but Titan anything is a waste of money, its really meant as a cash grab for idiots who don't know any better and for people who want to use it as a workstation and gaming card combined. Really though 3 cards is a waste in pretty much any scenario, for it to be worth it you'd need to be at a massive resolution.
I would say neither. 2x 780 ti would be the right choice since it won't be much of a difference between this sli config and two titan blacks. If you really want to play it safe, get 2x 780ti 6 gb and you will have all the performance you will need. But if you are set on one or the other: go with 2x titan black. Most games just don't scale well to three cards making it a massive waste of money.