The 970 out performs the 780 in all aspects, with the other positive reasons stated previously:
but ot emphasise, it has a 15% higher memory clock speed than the 780, 40% higher pixel rate, clocked at
59Gigapixels/s, and 8 more render outputs. Also, it is capable of around 10% overclockability on top.
SO I'd bang my buck on the 970.