Don't bother with the ruins on Cozumel unless you are just on the way there. My vote would be for Chacchoben with Ivan. Tulum is quite developed. Everything is excavated and pretty trampled. Lots of t shirt vendors. Beautiful, and the ocean setting is also wonderful. If you have time you could go to the beach (long walk down) also. BUT.... Chacchoben is in the first stages of excavation. The jungle around the area is wonderful, we saw flocks of parrots. Ivan's tour is fascinating, you will not be sorry. Check out all the web sites on both places and see which appeals more. Touristy fun or more authentic Mexico? (NOT bashing either choice!)
Carla