I agree with TC, the tour to the ruins is long and tiring. We took the excursion to Altun Ha in Belize and were so sorry we did. The bus ride was tedious, over horrible roads with huge potholes...the restrooms were flooded at the site, the hordes of bugs started biting as soon as we exited the bus (thank goodness I had brought insect repellent), we were not allowed to climb or sit on any of the ruins, it started to rain and the ground turned into a mud slide...I couldn't wait to get back to the ship and had to face that horrible hour long bus ride back. Hated it!!!
As far as Key West is concerned, it's a really cool place with tons of stuff to do. I live in Florida and the water temperature in the Gulf right now is 56 degrees so plan accordingly if you want to snorkel or swim. I'm sure the water temperature in Cozumel is a lot warmer. Cool stuff to do in Key West in addition to the Conch Tour: visit Ernest Hemingway's house, the Pirate Museum, the Shipwreck Museum which has a 60' lookout tower, Mel Fisher's Maritime Museum, Harry Truman's Little White House, Key West Aquarium. There are lots of restaurants and bars sprinkled all over (not just on Duval Street). If you like Key Lime pie, take a walk to the Blond Giraffe on Front Street, yummy. Dante's on Caroline Street has some great fish tacos and pitchers of beer and a huge pool in the middle of the restaurant for patron's use. All of the above mentioned are within walking distance of the dock.
Cozumel has some good shopping and nice beaches...we have been to Paradise Beach, Chankanaab Park and Playa Mia.