Tulum will be a LONG day. Cozumel is beautiful and has so much to offer leaving it seems a shame. IF you think this will be your only opportunity to see Mayan ruins then go for it, if not I'd save it for a time that I was already on the mainland. Just my opinion. It is a beautiful place. Also the first place I ever saw a whale shark. Very up close and personal. Now they have tours just to go out and TRY to see them.