I think in winter, modularity is best. Managing your temperature is going to prevent you from overheating and sweating. Two pieces means finding that custom warmth, quickly, will be much easier. You can wake up cold four hours after you went to sleep, pull a secondary piece a little tighter, and go right back to sleep.
Just slept down to -10º a few nights using a 20º synthetic outer bag and a 32º synthetic inner bag. Total weight was a little under 5lbs, not super light but I don't have to buy another $400 piece to get under 4lbs.