Antalya is a far better bet for winter flights as it still has regular scheduled international ones as well as the domestic flights from Istanbul. Although Dalaman doesn't actually shut for the winter, it's really only viable for international charters and the domestic flights to and from Istanbul.
Granted, the journey from Antalya to Fethiye is an hour or so longer, but I still prefer that option for winter as there are plenty of flights for less than £350pp return.