Whether the GTX 970 has 3.5 or 4 GB effective it's still more than a standard GTX 780 Ti with 3 GB, so I'm guessing you have to run some rather extreme resolutions and AA modes to see a practical difference. In fact the latter will generally beat a 970 whether single vs single or SLI vs SLI at UHD (3840x2160) resolutions.
What I do know is that my 2x970 totally trashes a single GTX 980 at a 20% price premium as they do have 2x13/16 = 26/16 the shaders, both cards shut down the fans at idle so it's extremely quiet and even at full tilt both cards together pull just 2x145W = 290W. I'm kinda surprised nobody's done a single card version yet since it's still under the 300W ATX limit.
It runs games at 3840x2160 on a Samsung UD590 beautifully, even though it's a 1ms TN panel it's not for twitch gaming as it's 60 Hz on DisplayPort with no fancy sync options and 25ms input lag but it looks extremely good. And at monitor distances you can definitively see the upgrade over 1080p while the TV benefits are more dubious. There are better setups, but for being such a high-end system the price/performance was extremely good.