I was married last month at the Riu Caribe (approx 10 min away from the Riu Cancun and Las Americas). The three hotels share one wedding planner, Lizette Sanchez. She is absolutely phenomenal and will do everything she can to make your day perfect!
The Riu Caribe is the more laid back hotel of the three (older building--originally the Gran Caribe Real, bought by Riu Resorts in 2000, larger beach area, quiet hotel, incredibly friendly staff). It is slightly outside of the bar and restaurant strip (a $0.65 bus will take you there within 5-10 min), but very close to the ferry for Isla Mujeres. I had a beautiful beach ceremony (four post bamboo arch covered in hundreds of bright tropical flowers and tulle, an English-speaking officiant, a mariachi band, etc), a cocktail reception in the gazebo (champagne toasts and cake cutting), and an offsite dinner (sunset lobster cruise).
The Riu Cancun and the Las Americas are both very beautiful. The Riu Cancun has a small dance club in the main level of the hotel. We didn't like the fact that the sun goes down quickly at the Riu Cancun, and there is a lot of shade on the beach by early afternoon.
You will have a wonderful wedding at any of the three Riu resorts. Lizette will do a wonderful job!! One suggestion that I have is to bring your own photographer. We selected the package for 40 guests and ended up with two photographers--the hotel photographer (for 12 pictures--included in the package), and Citlalli Rico (phenomenal!). The hotel photographer pics are pretty basic (candids, colors and black and whites) and we're happy we brought in Citlalli!