I just wanted to share a different experience with the same site. We went to Tulim with
DCL. It was a 30 minute Ferry ride and 45 minutes on the bus, but that still left us with over 4 hours at the ruins site. The tour started on the bus with the tour guide telling us about Mayan culture. When we got there, the guided tour lasted about an hour, which was a perfect amount of time for information and seeing the sights, and that left over three hours to explore the ruins on your own, go to the beach (yes, it is a BEAUTIFUL beach), or shop. Personally, I didn't find the site to be nearly as pushy or commercial as some other places we went on the same cruise. The vendors were all a distance from the actual ruins sites and the beach, and they could be completely avoided if you chose, unlike a lot of places. Also, the tour guide had a wireless headset and we all had headphones, so we could hear him clearly without having to fight to stand next to him. This made it a lot easier to see and hear everything. I wish more tour groups did this. One criticism I had was that the included "lunch" was a ham sandwich from a cooler. But they did give each person two bottles of cold water, which were very much appreciated. It was a HOT day.
To the OP, I'd say it depends on your attitude and what you want from the trip. If you are super excited to see the "best" ruins, go to Chichen Itza. It is an even longer bus ride after the Ferry, but surely the biggest ruins site. For us, Tulim was a good middle compromise. I was worried at first that Tulim wouldn't be "big" enough, but for us, it was. It ranged all over and was quite beautiful.
Let us know how it works out for you!
Also, several news stories and Trip Adviser says that Chichen Itza has been closed to climbing since 2006. So you will be able to see it, but not climb it, touch it or go inside. I'm not saying it isn't still worth it. Just trying to set people's expectations correctly.