Lots of games don't support SLI. It's also trickier to compare performance in laptops because we don't know how they are throttling due to heat so a GTX980M for games that do NOT support SLI could be 15% to 30% faster.
(if I had to guess I'd say maybe up to 40% faster for 2x970M vs 1x980M but it varies a lot. also, the price of only $300 makes me question if everything else is apples-to-apples because it seems too little difference. for example, if the SLI didn't have GSYNC but the 980M did then it's an easy choice)
Also not sure about the rest of the specs though I strongly recommend your screen have these specs:
IPS
75Hz
GSYNC
Other:
I'm not sure if you can wait, but a GTX1070M or GTX1080M are going to be really awesome. The new die shrink means much less power so longer battery life (or smaller battery), probably less noise and thermal throttling.