Short: 970m is good enough
A bit more detailed:
The 970m got pretty much the same performance like a good desktop-GPU 3-4 years ago and should be good enough for high details at current games. The 980m got about the same speed (or a bit better) then the current GTX 960 (non mobile version) and is good enough for very high details at most games but still a bit to weak for solid VR (that would require a desktop version of the gtx 970, gtx 980, or amd 290 or fury). You could even “use” UE4 with something that got the speed of a 950m GPU but you would be motivated to optimize your game and implement some graphic detail sliders way sooner then others (which is probably no bad thing) ;).