We are doing both in June, so I can post when we get back! We are very interested in the Mayan ruins. A beach is a beach is a beach (and we'll have plenty of that at Castaway Cay) but the Mayan ruins are unique! DD is studying ancient civilizations in school this year, and she learned about the Mayans, so she is extra excited.
FWIW, we have 4 ports on this cruise, and we are spending 2 of them at Mayan ruins. The other 2 will be water/sand/beach days. IMHO that's a good mix.
My family has done both sites on different cruises. Tulum is large with more buildings and the beach. It is a beautiful place, but more touristy. Chacchoben was more interesting to me because the area is not as developed for tourists as Tulum. We really enjoyed both locations, but I don't know that I would have done both on the same cruise.
Any safety issues with either of the ruins. I am scheduled with DD9 to do the Costa Maya ruins but my wife is scared about news reports about Mexico.
If I could only pick one, it would definitely DEFINITELY be Chacchoben at the Costa Maya stop, and only with Native Sons. Ruins are fascinating to me anyway, but the passion David & Ivan have for their history and their culture, and the knowledge they have as natives elevates the experience to a level I can't imagine the big tourist-y places being able to achieve.
Out of 7 cruises so far for DS & I, that day was the best so far for both of us, bar none. We can't wait to finally go back again next year!