Before you buy, ask this......do you use it as a pack as well. That will help you decide on size......what do you normally carry in a pack?? Also, the 2 types are air and nitrogen air is an easy refill....nitrogen is a trigger replace and a bottle refill, at around $100 a pop....Klim has a new bag with a built in fan that eliminates the bottle refill thing, and its up at around $1400. Polarisrider's bag is a good one...had that one on my back for 5 years.......if you go used just do the blow up test, or ask the owner to do it prior to purchase.....the idea that you are going to a bag is the bomb and I thank ypou for that.....