Both are great locations, but I know a lot of people that run into "some" issues with RIU, however with that said, I still think it's a good choice. Dreams is amazing, so i would lean towards having your wedding there.
OH and if it makes a difference, RIU is on a beach you can swim in, Dreams is not. Also RIU is closer to town if you wanted to shop and go out for some night life. Dreams is closer to where you can surf. (that's a huge plus for my hubby)
Best of luck and happy planning