I’ve decided that I need a NS Surge frame in my life and I’m a touch perplexed as to what size to get! ” 15.5” (fit’s like 17” MTB), 17″ (fit’s like 18.5″MTB)”

I’m just under 6′ and this bike is going to be aimed much more at fun going downhills than going up them, does this mean that I want to be looking at the 15.5″ then? I really don’t want to end up with the frame being too small, or too big really!