The "curse" I had was broken: I could finally visit Chichen Itza.
I highly recommend visiting the site on December 31: there were less tourists than at other times, and it is less hot. Adrian, our guide, said the worst time is the day of the spring equinox, which attracts thousands of tourists trying to see the shadow of the descending snake on the stairs of Kukulcan pyramid.
We were surprised at the amount of vendors who sell souvenirs in the site. In all other sites in Mexico, these vendors are OUTSIDE, not beside the pyramids, IN the site. They were funny, though. Most of them are Mayans, they are always smiling, and some say silly things like: "Come here, I will steal less from you", "Come buy my Mayan souvenirs made in China", and the sort.
Our guide did not agree with our Coba guide. According to Adrian, Chichen Itza was by far more influential than Coba. We need to ask this question to a qualified historian.
All I can say for sure is that both sites are very different. The forest in Coba is thicker, and trees are taller. There is more walking in Coba between the different buildings, their style is more "Mayan" (as far as I know, which is not much), and they look older (maybe because Chichen Itza has been restaured more thoroughly).
There is a lot to be said about Chichen Itza.
For now let's just say: ¡Próspero año nuevo! Happy new year!