That depends whether yer in that bag alone or with another camper... If you're alone I think you'll be warmer in long underwear so you have a warmth layer that stays next to you. Nylon gets cold and when ever you roll over its cold on your skin. Long underwear will go a long ways toward keeping body oils off your nylon and keeping the bag way cleaner in the long run, and cleaning any sleeping bag is about the most destructive thing you will do to it except maybe putting it in a compression stuff sack. Oh yes, if you have to get out of that sleeping bag naked you'll be much colder. Often in winter I take a lighter sleeping bag and sleep in a down jacket and down pants of bibs, so when I get out of it, I'm warm.
Just my $.02
Jim