My personal preference is Akumal. It's much smaller, more laid back and quieter. It's right in between PDC and Tulum, about 20 minutes drive each way. From either place you can do cenotes, take a day trip over to Coz, or do some ocean diving out of either one as well. The ocean diving in Akumal was ok but not spectacular. Avg about 75ft, nice reefs, saw baraccuda, big turtles, the largest morays I've seen, shark, rays and the usual assortment of tropicals, coral, fans etc.
If you've looking for accommodations or general info,
www.locogringo.com is a good resource. You can book condo's, villas, houses, hotels through the site.
In Akumal we've stayed at Vista del Mar
www.akumalinfo.com (condo, not hotel, the htl rms are very very small), and did ocean dives with Akumal Dive Adventures. It's the same owners so if you prebook a few dives you get 15% off on the dives, and 10% off on accommodations.