Fly into Dalaman and you can turn either way. Head east and within an hour you are in Fethiye; head west and a little over an hour and a half brings you round to Marmaris. Because the airport is the same, the decision is not about flights at all. It is about what you want to see from your balcony and what you want to do once you have unpacked.
Put very simply: Fethiye is the choice for people who want a base for exploring, with something new to see most days. Marmaris is the choice for people who want the bay itself to be the holiday, with boats, a long waterfront and plenty going on after sunset. Neither is the right answer for everyone, and plenty of travellers would be happy in either.