Chichen Itza or Coba?
 

If you only had enough time for one of these places...which would you choose?

Where are you starting from? You could stay in Playa del Carmen or Tulum and visit Coba on a day trip. That's what we did (several years ago). Chichen Itza is not a day trip from Playa del Carmen (or Cancun) in my opinion. So where you be based?

We will be staying Playa del Carmen. I have heard that I could do Tulum and Coba in one day, which is cool because I want a full day at Xel-Ha (not 1/2 day) but if I only have time for either Coba or Chichen Itza....I have heard there is more to see in CI.

I have been to Chichen Itza, Coba and Tulum. More ruins have been excavated in Chichen Itza BUT it would be a much longer trip from PDC. The ruins at Coba are extensive but not everything has been fully excavated. Tulum has the most beautiful setting (right on the ocean) and is very compact. Yes it is possible to do both Tulum and Coba in a day (you will not see "everything" at Coba - it is very spead out - rent a bicycle or pedi-cab to get around at the ruins). Personally I found Coba more interesting and less touristy than Chichen Itza (but I am a history major and things that fascinate me might not fascinate you). I would choose Tulum/Coba over Chichen Itza because:
proximity to PDC
2 ruins (very different from each other on very different sites)
Coba is considerably less crowded than Chichen itza
For me Coba is more interesting

I'll weigh in for Chichen Itza, one of the most impressive Mayan ruins, I thought it even topped Tikal in Guatelmala in terms of how much is excavated and what there is to see. If this was my only chance to see a Mayan ruin, this would be the one.

It's been a couple of years since I visited Coba, at the time I think the estimates were that only about 5-10% had been excavated and having already visited Chichen Itza, I was less than awed although it was interesting and quite peaceful.

Chichen Itza can be done on a day trip from Cancun or PDC but it's a very long day and if you take an organized excursion you will arrive around midday (bus ride is 3-4 hours) when the sun is the hottest and the crowds are the largest. Bring sunscreen and lots of water if you decide to go.

Palanque is better than all of them, but too far from PDC for even a long day trip.