Coba & Chichen Itza
 

Does anyone know how long the drive is to both of these ruins from Playa Del Carmen and Tulum?

Coba is much closer, only an hour to hour and half from PDC and less than an hour from Tulum.
Chichen Itza is three to four hours from PDC. Rather than driving there I took a flight tour which took less than an hour to get there. This was a highlight of my trip since we had the ruins alsomt to ourselves. The flight arrives by 10 am before all the tour buses from Cancun get there (around noon).

Ok. I've found out that I will go to Coba from Tulum. But which is closer to Chichen Itza, Tulum or PDC?

PDC is a bit closer but not necessarily faster since you have to go through Cancun. From Tulum you can cut over to Coba and then north and west, bypassing Cancun. Chichen Itza is a long drive from either. If it were me I'd either fly or do an overnight trip, particularly in the Spring or Summer. The jungle is stifling hot.

Tulum is closer if you go by the Coba road - it is also toll free. If you drive north from PDC through the Cancun area and then head west the toll road is pricey but pretty empty. The drive north from PDC to Cancun will take you about 40 min. Then you head west on the toll road. The drive from Tulum to Chichen Itza is about 2 hours. Leave early. If you go to Coba from Tulum it's around an hour on the same toll free road to Chichen Itza. The road has a roundabout which you go through and then head south to Coba. They are two very different types of ruins.