I am sorry my friend but I am aware of the cuda cores and still doesn't make much of a difference! You will NEVER use more than 3 Vram in any monitor and game setup (well you might in 6 years but that's a whole different storry)! The 8gb of vram are mostly for people That have like 5k cameras and wan't to downgrade to 4k,2k and 1080 at the same time or some other video crazy stuff that takes a lot of vram and needs a lot of shading/texture mapping units. What games are needing though is clock speed and vram clock so it can process less things faster. It's like comparing a server-cpu with a gaming cpu only with a bit less difference. You can do really good gaming with a 4 core cpu that has a clock of more thatn 4 hz and you can do crappy gaming with a 16 core cpu with like 2,6 hz or so.
It's really not thaaat of a huge difference between the two GPUs but for gaming the GTX 970 has the upper hand.