If you are in Cozumel until like 11:00 p.m. etc. then you can do both - take your tour once you're back to the ship grab a cab into the city. If your leave about 5:00 ish then it's one or the other. Just depends on what you want to do, the ruins in Tulum are spectactular and if you want the culture of Mexico, etc. then Tulum is it but if you want relaxing fun, fun, CRAZY fun Carlos and Charlies with walking around town.