We did the Chacchoben tour on 2/25/06 cruise - best excursion of our trip - took about 4 1/2 hours, so still time to do other things during the day (we docked at about 10 am, tour went from noon to about 4:30, ship left port at 5). Our guide was superb (Esbile was his name). The ruins are newly discovered, and only partially excavated (but there is a lot to see), so you really feel like you are going back 2000 years in time touring them.
Much of the ruins area is covered with trees so it is shady most of the tour - this could be a big factor in August when I suspect it will be quite warm there.
We were scheduled to do a ruins tour in Cozumel, but the ship did not go there due to the hurricane. It sounded like there were other things to do at the Cozumel tour (beach, etc.), though it was longer (7 to 8 hours). There is no beach or other attraction at Chacchoben, but there is a nice beach right at the dock in Costa Maya.