That depends on the type of food you want to prepare. For any food that contains carbohydrates (e.g., veggies), low-heat cooking is healthier than using high heat because it avoids the formation of advanced glycation end products (AGEs).  Using low heat also retains more of the micronutrients, including vitamins and minerals. As a result, I’d argue that a steam oven that uses lower temperature settings than an air fryer is the healthier option.