Is this your first trip to Playa? One thing about Playa, once you experience it once, you WILL come back. It just has that effect on people. You could always save one place for your next trip. If you only have a week, then 2 days for "excursions" really cuts into beach time.
I'm thinking about doing the CI trip on my next trip in Feb (wife has never been), but if I was trying to see both on a single trip, I think I'd head to CI early one morning, then head to Valladolid after and spend the night. The next morning I'd head to Coba and then probably hit Tulum after before returning to Playa.