HI! I just got back from the Riu Palace Riviera maya. It was gorgeous. I am thinking of chosing the Iberostar paraiso for my wedding. The Riu is my second choice, but i want you to know that the main reason for that is that the Riu was a bit more relaxed and quiet. My fiance has siblings who are teenagers, and I want them to be able to be involved in the activities that the Iberostar has. I've stayed at Iberostar Paraiso beach, and it was a little more active than the Riu. More pool games, activities, more participation at the night shows, etc.
The Riu's packages are cheaper though...and I will tell you that I saw 3 weddings there and they were great. I totally would go back there in a heartbeat, and if the Iberostar falls through, I will be going back to RPRM!