I think either are very reasonable but very different choices. They can both help your app but in different ways. As others have said, I think being an EMT really prepared me well for med school and I would definitely do it again. But in many areas EMS is very poorly compensated - assuming you do have to eat and pay bills during your gap year, it's not unreasonable to choose the option that has better benefits/allows you to have the most flexibility in other aspects of your life. If both are equal, pick based on the type of work that seems enjoyable. Working in a clinic with regular hours and a more routine schedule/workflow will be different than possibly working long/late/weird hours, but also getting to see and help with some unique and high intensity situations.
If working directly with physicians is really important to you, Ophtho tech will be the better choice (unless you can find and ED tech gig). But EMS will give you other important skills and experiences that will be more directly related to what you do in med school. Ophtho itself is a very small part of med school for most, so some skills will be less transferable to other specialties, though as a retina tech I imagine you'll have a leg up on your classmates in doing eye exams lol
