To my understanding the GTX Titan is slightly better overall. BUT you'll be paying a lot for a very small improvement compared to the GTX 780 (i think its like $1000 vs $650). I can't find many benchmarks but here's one http://gpuboss.com/gpus/GeForce-GTX-TITAN-vs-GeForce-GTX-780. I use vurts flora overhaul, the lite version of Skyrim hd 2k textures and some other texture mods and i've never seen my 2GB gtx 660ti use more than 1.6GB of vram when running Skyrim. But i'm not sure how much more the mods you wanna run will use.
