690 all the way but if you are willing to go SLI on both of em then Dual Titan SLI is a MUST!!!
Check it out your self (you'll see Titan in 2x SLI performs 20% or more than the dual gtx 690)http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_Titan_SLI/
A single 690 can even out perform 690 sli because when you put 690 into sli it means the game reads it as a Quad SLI which 80% of games don't support so it means the Frame rates go down pretty badly(the more GPU there is and if that game don't support multi GPu then the Frame rates will keep going lower)
Go look at the website and youll see the Titan SLI is so much better than 690 SLIhttp://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_Titan_SLI/
but for single for the 690