I am struggling with the exact same decision myself!! Riviera Maya vs Punta Cana...I go back and forth. The prices look about the same based on my research so far.. just having problems narrowing down the resorts. I've done lots of reading of this awesome forum and it sounds like picking the location is the hardest part!
As far as the violence in Mexico, I was concerned with that, too, but in looking at the US State Dept's website, there doesn't seem to be any problems at the Cancun/RM area related to the drug trafficking issues. Just your normal be safe in foreign countries kind of thing, like don't go out late at night esp by yourself, etc. So I wouldn't let that stop you from considering the area.
I don't know much about Dreams, but all the wedding-type reviews I've read say that they do a GREAT job with destination weddings. Do a search and you'll find LOTS of Dreams brides' posts.
Seems to be great AI resorts at both places, just a matter of what works best for you and your guests!