I'm in the same boat (cruising humor) as Penny. I have been to Tulum and what Penny said about it is true. I have not been to Coasta Maya yet, so I can't say anything on Kohunlich (actually have not heard of that one until now, so maybe that says something?) The beach at Tulum is a must see so if I had to choose for you, it would be Tulum. Yes it take most of your day, but assuming you will be back to Cozumel in the future (we have been several times now) then using you port stop time up is not such a big deal.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007