Depends on other interests and the time you have in port. Tulum is a shorter distance - it's the only set of ruins right on the Caribbean, which makes for a neat setting, and they often include other activities with the Tulum tour, such as snorkeling, or visiting Xel-Ha (water park), etc. Chichen Itza is most of the day, and a long bus ride away, but is bigger and more impressive. Both tend to be crowded, perhaps Tulum more so. Hope that helps!