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Why?
780ti wins in benchmarks.
970 is probably the better option as it can run DX12 and therefore more useful for future game releases.
I Wonder if the higher 384-bit Memory Bus of the 780ti over the 256-bit of a 970 can also make some games run smoother ?Depends if the game or benches used the VRAM or Bus Width more which wins that result.
What is your current graphics card?