Personally I would go for
one of these.
Re. overheating, the usual thing to do is to keep it unzipped (same as your three layers of clothing underneath). Granted, if you go down fast (or while asleep) you might end up in quite an unpleasant situation, but there is a balance to be had between dying of cold shock/hypothermia in an emergency and dying of heat stroke in the normal course of events
