I've been to both in the past year, and agree pretty much with what's been said.
In Puerto Vallarta, you can easily take a taxi from any of the hotels in Marina Vallarta or Nuevo Vallarta into town for the night and it's pretty cheap. From Tulum or Mayakoba into Playa del Carmen, not so much.
The beach in Playa del Carmen itself is not particularly good. It's not that deep, and is very party central. The gay-ish beach, Mamitas, is like a Miami/South Beach scene. There is one gay club there, which was not so exciting. I think Playa del Carmen's days as a chiller Cancun are over, and it is now supertacky and filled with tourists. Overall, I think Vallarta provides more options. I have some pics and videos of Playa del Carmen, including Mamitas and the gay club, in a post i did from my trip there in February.
Playa del Carmen and La Pasion Hotel Boutique ? You Went Where???