If you remember one thing from this comparison, make it this: the two places operate at different sizes. The Antalya region stretches along the coast in both directions from the city, with resort areas such as Lara, Belek, Side, Kemer and Alanya, and many large all-inclusive resorts. Fethiye centres on a single harbour town, and its hotels tend to be smaller in scale.
Neither is a drawback in itself. A large resort gives you pools, restaurants and entertainment without leaving the grounds. A smaller hotel in Fethiye or Ölüdeniz usually means you spend more time out and about, eating in town or heading off on a boat or a hike. Think about which rhythm suits your group before looking at anything else.