I am slanted on this subject, as I love to go to Cancun. I am actually going there on my honeymoon in June. My parents own a travel agency, so hopefully I can give you some decent advice:
It depends on what exactly you guys want. If you are younger and you want to party then go to Cancun. If you are older and you want to party then go to Miami. If you don't care about partying at all then go to Cancun.
Cancun is probably the most relaxing place I have ever been and one of the best parties I've ever been to. If you are 17-20 years old then it is a really fun party in Cancun. It can still be fun if you are older, but if you are looking for a party and you can drink legally I would go to Miami.
Cancun has some of the most beautiful beaches, amazing hotels, and lowest prices. You get the most bang for your buck at Cancun.
That being said, you can't catch a Celtics game in Cancun.
Let me know what you are looking for in a place and the age of your buddies and I'll try to give some more advice.