At 1920 x 1080 a GTX 780 will max every current game including Crysis 3. A 780 Ti would be more futureproof but it's not necessary right now unless you are going higher resolution. I just got an EVGA GTX 780 Superclocked and out of the box it's faster than the Titan is. A single GTX 770 will max most games but not all at 1080P.
I think I'm going with the EVGA GTX 780 Ti, as you said it's more future proof and I always have the option of combining another in the future.
Only question remaining is, will a single EVGA GTX 780 Ti perform on par with the 770 GTX SLi?