LOL, you'll get as many different responses as we have fans of each restaurant. IMO, it often depends on what you order, so while one person may have a fabulous dinner, another may be left disappointed. At Miramar I've had fantastic ceviche, whole fried fish, great fish tacos, seafood soup that was like a stew, garlic fish filet, and some of the best guacamole ever. At Brisas Grill we split the tikin xic special and it was unbelievably good. Even the veggies had a nice charcoal flavor and were perfectly done.
Others rave about different places, and I'm sure those are good too. In all honesty, I've rarely had a bad seafood dinner on Isla. At worst it was just bland and ok instead of really excellent. Can't speak for other dishes at either of those places but what I've mentioned have all been terrific.