I think if you book the Palmerus, you get your choice (we had to pick between the grill and the beach at least 4 hours a head of time), once you pick it - they're both yours though! That was another awesome part of Palmerus - if you booked the beach, the grill was the back up.
Mariachi Grill I think looks very similar to the Palmerus - it's further down by the Playa Azul. Maybe you could see about having the ceremony there? I honestly didn't really check it out. I think they make the grills look pretty nice though, whichever one you get.
The only one you normally can't get, is the Coral Grill. For one its HUGE, you would need a big wedding to make it look decent. It's also the Japanese restaurant most nights.
Your planning thread is awesome, what a beautiful wedding Avens!
Does anyone know about the Mariachi Grill? also one of our ceremony options but just wondering about the beach in that area. We would like to have the ceremony on the beach if possible...as with Avens' at the Palmeras beach...thanks!