970. Even in the games when the 780 Ti wins, the 970 usually isn't far behind and this is a rare comparison where the 970 actually has a VRAM advantage. And the architecture advantage since Kepler gets less attention from Nvidia and is less power efficient.
Performance might not be different on average between the two at the moment, but it's like to continue to swing towards the 970 in the future with newer games. Given the lower power consumption, added features like newer codecs and HDMI2.0, and slighter larger VRAM, at the same price I'd definitely go with the 970.
When the Witcher 3 came out, I had a 780 Ti and my spouse had a 970. His 970 was blowing my 780 Ti out in performance, which was shocking to me. The extra vram along with Maxwell is just better to go with imo.
As games utilize more and more compute resources, the Maxwell architecture is pulling away from the Kepler architecture.
While the GTX 780 Ti has a higher theoretical compute flops count, its arithmetic latency coupled with its inferior utilization of available compute resources hinders its performance. It's the same reason we've been seeing the R9 390 pulling away from the GTX 970. The R9 390 is a superior compute card than both the aforementioned cards.
Just keep in mind 970 will likely have the fate of the 780 once Pascal releases in a quarter or two... they need to make game-specific optimizations to keep it competitive, and I doubt nVidia will continue to support it well once Pascal is out.
As soon as Pascal appears thes two GFX will end up at same performance with the chance that 970 will suck if no specific driver adaptions are made for it. Considering the Care that is currently done for Kepler after Maxwell Launch you should not calculate too optimistic.