My wife has decieded she wanted to buy me a sleeping bag before my up coming trip to take Mor K. Class in February, which everyone knows gets pretty damn cold. Up until now i have used a 20 year old Northface Catsmeow 20 F bag, then for colding i put a 45F marmot my wife uses in the summer, and then a gortex military bivy. But i have the option now to buy a sleeping bag that i will be using for the next 25-30 years i assume for all my winter trips and expeditions. I do plan on making trips up to sweden and norway in the future, and more cananda trips. And possibly bigger alpine trips as my mountaineering skills get better. So 1 bag to rule them all for winter trips. My question is how cold should it really be rated? They have some awesome deals with 20% off at backcountry and its a given that i want to go with a down. But we were looking at the Marmot 0F Lithium. Or should i go -15 or -20? Is it really necessary? Thanks for the feedback guys im hoping to order this soon while the deal is going on!