Personally I would get the L, its going to do more and if you decide you want a shorter can later down the road get it then.
Better to have quieter and longer than louder and shorter.
I have a L on my POF P308 16.5" AR. It works great, it's just that it's pretty long, and weight...well let's be serious, I have a 16.5 AR10 so weight is already phuqed! ;-) That's why I submitted the paperwork on a S last week. Plan on putting that on a SBR Revolution when they come out later this year.