Really, like dressing for variably cold climates, it is best to go in layers. Acute, short term survival issues are going to differ greatly in how you respond vs. chronic, long term survival needs. I think preparing to respond at a range of conditions is most appropriate, long term being anything that is going to require self sufficiency for more than 3 weeks. By then you should be converting your MO from "what else is in the bag?" to "what's in the pantry?"