We just got back from Cancun, and spent three whole days on Isla Mujeras because it was exactly what we were looking for (quiet, uncrowded, North beach is better than Waikiki IMO, without the hoards of people.) We actually would call home from a payphone on Isla because it was quiet and we could hear! We could easily spend a whole week on Isla.
We went to Playa for a few hours and were frankly done with it just that quickly and wanted to leave. Crowded, Americanized or Europeanized, not sure which. Things cost twice what they did on Isla (pottery and stuff like that). I would stay at Playa if I were doing the ecoparks or to go to Cozumel but not for any other reason.
Chichen Itza will be a full day from either place. The ferry from Isla is a snap, $7 round trip, not a big deal at all. (we did it 3 times)
Have a fun trip!