Cancun is very much like seaside heights. Can be very appealing depending on your personality, interests and budget or could be a jumping off spot.
I've been twice and feel I never need to return. Tulum is about 90 minutes south of Cancun and a far more intriguing place in my mind but that also depends on your personality interests and budget.
How long are you going for?
What do you want out of this vacation?
Are you traveling with people?
Already arranged some travel?