I'm 5'7" 155lbs. I ride a NS system 151. I like having it that short because I skated for almost a decade and I'm used to being on a shorter board. I also like how I can really whip a turn hard when I want to. In the really tight trees where you don't have the room to make mistakes, the shorter board is gonna be your savior.
the cons: I feel every damn lump and snowball that I ride over. It's easy to get pitched off your line if your going fast, but I've adjusted. O yea and of course, it's gonna make your legs hurt a hell of a lot more in the powder because you're really gonna have to lean back.
I'd say for you, roll with the 155 since you said you do mostly free ride now, but when you head to the park, it'll do just fine for you. especially the SL-R, I'm getting one in the next couple weeks too