I went with the large, works GREAT. I used it with a t-shirt and polypro baselayer (if needed) in the summer, and for winter bring along a heavier coat to put on top. SUPER warm, and amazingly compressible, plus about a pound lighter than the fleece hoodie I was taking before (and I don't have to strap it to the outside of my pack). All in all, very happy with it and the deal i happened to find.
I just re-stumbled upon my own old thread and thought I'd share =/