You're going to have better nightlife in Cancun based on the name alone. That doesn't mean that nothing is going on in Playa. There's always something going on. There's a lot of hostels that have their big nights on Mondays or Tuesdays so you can always go there to at least meet some people and see what they're doing that night.
Cancun is going to be Americans. Playa is going to be a lot more diverse. Cancun is going to be expensive. Playa can be cheap if you look around. And don't forget, they're only an hour apart by bus so if you really hate it in one place you can just hop on the next bus.
Also, don't sleep on Tulum. I didn't make it there but it supposedly has really nice beaches, and is probably worth a day trip at least.
Cancun is going to be Americans. Playa is going to be a lot more diverse. Cancun is going to be expensive. Playa can be cheap if you look around. And don't forget, they're only an hour apart by bus so if you really hate it in one place you can just hop on the next bus.
Also, don't sleep on Tulum. I didn't make it there but it supposedly has really nice beaches, and is probably worth a day trip at least.