Sweet setup. What I'm about to say makes me cringe, because I hate when people don't give me direct answers, which is what I'm about to do. You most likely would be better off switching to 980 Ti's. For the $1,600 price of a Titan Z on NewEgg, you could purchase 2 1/2 980 Ti's and have greater performance. Also be aware that the Titan Z occupies three slots, unlike most other cards that occupy two.
But between your two choices, Titan Z or 2 x 690's in SLI, I'd stick with the 2 x 690's in SLI. But I've never had possession of either of those cards, so you're in a better position to make this choice.