go for the 970m, it is genrally better in performance
now
1. games of today are requiring higher amount of VRAM , so genrally 6GB it better(but an overkill for today)
2. But the 970m is more powerful
3. for games of present day 6gb is an overkill , 3gb is more than enough. But who knows in 3years where it will be.the PS and xbox have 8gb vram (i might be incorrect)
4. even f you run out of VRam your ordinary ram will be converted into vram (though less effective) in win 7 +
5. req today are bloated up (ex. shadow of mordor doesnot need hat much VRAM)
6. if you donot have enough vram you must reduce the AA and texture quality(or resolution) ,but this is not a prob with 3gb cards now
hopwe this helps!!