Not sure what you mean by "gay friendly". BF and I have been to Cancun 7 or 8 times. Always stayed in one-bed hotel rooms (Sheraton, Westin, Melia, Le Meridien). Always treated with respect. There are a few bars in town.

Both PV and Cancun are very different, however. PV has a much larger gay scene. If you're going for the clubs, and want to meet some locals, go to PV. If you're going for the beach and only want to see Americans, go to Cancun.