Barbiegirl 27,
I completely understand what you are going through! I'm a little bit of a perfectionist and I want everything to be perfect especially for my wedding. We ended up choosing Riu Palace Las Americas. I'm about to do a site visit in April just to make sure! My dad wants to do a father/daughter trip before I get married, so he asked if I wanted to visit the resort before! We are def. going to stick with the Riu Resorts but not sure if I want to keep it there or switch over to the Riu Palace Riviera Maya due to the location. The following are some of the reasons for picking the Riu Resorts: it's affordable for guests, the wedding packages are affordable especially for large groups as we are having a large group, the option to do a reception, etc.
Dreams Resorts are amazing. I don't think you could go wrong. I was torn between Dreams Resorts and the Riu Resorts. My FI actually proposed to me at the Dreams resort in Nuevo Vallarta and it was a "dream." We picked the Riu resorts over the Dreams just because they are more accommodating for larger groups.
When I did my price comparisons the Dream resorts and Riu resorts were similar (e.g the rates for Feb 2010 from Feb 9-16 the rates were the following: Riu Palace Las Americas @ $1, 213, Riu Cancun @ $1,000, Dreams Cancun @ $1,200, Dreams Puerto Aventures @ $1,200, and Dreams Tulum @ $1,300.
Hopefully this helps. Let me know if you have any other questions.