I use the HSS also. Easy to put on. I have the newer mesh vest type and the reversible orange/cammo one. The mesh net style is a bunch cooler in ow season. I'd get the large. By the time you add in the bulk of cold weather clothes, the large I think would be more comfortable,

A couple of buddies of mine use the "Seat-of-the-Pants" system and like it. Both are the same idea, just designed a bit differently.