Sizing question: male, average build, 5'8,...
Sizing question: male, average build, 5'8, 165 lbs - do I get S/M or M/L?
It really depends on your torso length. If you have a torso in the 18 or 19"range or larger, get the M/L. Anything shorter, go S/M.
I would go with the S/M. I am 6'0, 200lbs and the M/L fits well. If anything, it's not small/snug enough.