The ruins at Tulum are far more extensive than those at San Gervasio (on the island of Cozumel), but the ruins at San Gervasio are in some ways more significant because the island is a holy place in the Mayan religion.
The tour to Tulum is fascinating, but it is a full -- and I do mean LONG -- day. If you opt for this tour, you will take a ferry from the San Miguel de Cozumel to Playa del Carmen on the mainland, followed by a bus ride from Playa del Carmen to Tulum. Of course, you will reverse this at the end of the day. Be sure to bring at least one large bottle of water per person from the ship and to carry it with you into the ruins, as there are no refreshments within the ruins and the weather there tends to be hot (upper 90's in March!). We had one person in our party pass out within the ruins, probably due to dehydration, in spite of the tour guide's efforts to keep our group in the shade. Nonetheless, the ferry and the buses are air conditioned and you will have plenty of time to explore the ruins on your own after the guide gives an excellent explanation.
If you don't want to invest the whole day in this tour, the tour that combines the Mexican Folkloric Show with the tour of the ruins at San Gervasio is an excellent alternative. The folkloric show in San Miguel de Cozumel is the best that I have seen anywhere.