The 970 has a small but pretty consistent edge over the 780:
http://www.anandtech.com/bench/product/1355?vs=1036
Depends on price as the 780 is still capable, but 970 is the better card. Unless you're aiming for multi-monitor or 4K gaming, I suggest the RAM issues with the 970 have been somewhat over-hyped, and it'd still be better than the 3GB RAM on the 780 anyway.