I know it's an old thread but I'd like to say, a Gigabyte Windforce 780Ti OC 3x vs a Gigabyte Windforce G1 Gaming SOC 970, the 780Ti comes out on top 99% of the time, even at 4k and if you oc the 970 to its limit, the 780Ti is faster. In games at 4k games use more vram if it's available, I've noticed, yet with the 780Ti they use less vram - go figure. Crysis 3 has the 780Ti streaking ahead of the 970 at 4k, the same with Metro Last Light. Project Cars, they are similar with the 780Ti being 3-5fps ahead. Both chuck COD:AW over 60fps at 4k with the Ti hitting over 10 fps more than the 970. I have both and also both in sli. I've not benched 1080p as it is a dead resolution in my eye's. I've also not benched using AA as that is pointless with 4k pixel density, but all other settings are at their maximum. Also, the 780Ti will output 60fps via HDMI and at load their wattage use is very similar. TDP is thermal output not power use. Both remain reasonably cool in SLI with the 970 winning by 8c.
The only reason to buy 970's over 780Ti's is for possibly better DirectX 12 support later on down the road, otherwise I'd easily go with the 780Ti over the 970. Believe me, the 500Mb of vram at present makes no difference in performance and for DirectX 12 games of the future, we've been lead to believe that SLI vram will stack so, no real vram issues there either. Second hand a 780Ti is upto £100 cheaper than a 970!