We were in Cancun last winter, Beautiful place. We took a tour to Tulum, the ruins were pretty spectacular when attached with the history of them, very knowledgeable guide and great sense of humor. Worth the trip unless you have time to see larger ruins in other areas. View of ocean is spectacular. Note: Bring your own bottled water and toilet paper, no water sales( very hot area) and they try to sell you squares of t.p. when you use the washroom. You can take a small shuttle to ruins for a fee or do a ten minute walk, the walk is fine. Tulum is worth seeing!!
The ruins in Costa Maya are fairly recently discovered and allow for more exploring but both are great. I did Tulum many years ago and loved for the beauty of the sea. The best ruin is Chichen Itza but that's not a choice due to the distance. The history of the ruins makes it all the more interesting and if possible I would do both. Another decision breaker would be the fact that in Costa Maya there is not much to do so I would definately do the ruins vs Coz there are so many more options and I would take advantage of that (that's what we did when we had Coz-Costa on our iten). Happy sailing!
Hopefully you will get to go to one of them. We have gotten cancelled on three times. Two times we did not dock for some reason or the other and this past November there was not enough people signed up for the tour. I told my husband that this must be an omen. They will probably fall if we ever get to go. Anyway I have heard they are both OK but most days are very hot and the ride there is very long.