Cancun resorts are more expensive and crowed depending on the tourist season. What I like about Cancun is checking the Mayan ruins and the ecological-parks.
Punta Cana, prices are more attractive. However, the historic town with beautiful architecture is 3 hours away..
Both places offer great beaches and resorts..It comes down to the kind of experience you want to give to your guests and your budget. answer this question to yourself. If you are invited to a destination wedding and as a guest you would like to turn the weekend wedding into a great vacation, what kind of activities you would be the most interested?
Keep in mind that some people love just seating at the pool reading a magazine or having drinks all day long. Others, are more into exploring the area or having both adventure and beach time..
I recommend you to look also into Puerto Vallarta in Mexico or Puerto Rico in USA...