Sweet!
Caye Caulker is basically a smaller/slower version of San Pedro, but equally cool in its own way. Amigos Del Mar is a good dive-op based in San Pedro. Diving in the Ambergris area is very good; diving in Turneffe is better. The diving in the area is pretty easy, with no current. However, these areas of Belize are spread-out over a much larger area than the Cozumel/Playa/Tulum triangle. A trip to Turneffe from San Pedro by boat, via Caye Caulker, is about 2-1/2 hours one-way. At the time, I believe a two-tank out to there cost a couple-hundred bucks per-person.
Diving between Playa and Tulum can be pretty good -to- just okay. The reef systems are not anywhere near as cool as Cozumel, yet there seems to be bigger animal life. Most of the diving is of the drift variety, but generally not in as strong a current as over in Cozumel. Surface intervals spent on the boat can get pretty choppy sometimes. Tulum is cool, and the beach there is spectacular, but there isn't much else going on. Tank-Ha is a good op in Playa for ocean diving. Diablo/Aquanauts, in Puerto Aventuras, is good for the cenotes and anything based further south.
Overall, Ambergris has better ocean diving than the area around Tulum, but the Yucatan has the cenotes (awesome experience, but you need good buoyancy control skills and know how to frog). Your trip already sounds like a win/win. So, either way, I doubt you'll be disappointed no matter what you choose.
Hope this helps, and have a great trip!