FWIW, I'm a cold sleeper. I own a WM Summerlite with 2 ounces of overfill; with no additional clothing I'm comfortable into the 20*F range. In your case with the Megalite vs Alpinlite, 2 ounces of overfill in the Mega would get 1#10oz vs 1#15oz in the Alpin (size reg, I know); save yourself 5 ounces, more versatility? I would. Re: concerns about one or another being too warm, they are continuously baffled bags, so you can shift excess down to the bottom if needed. More or less down in the bag will not extend the life of the bag; just care for it properly.
