Atoll diving is cheaper from CC than from AC. Local diving is easier and quicker from AC. There are more dive centres on AC, with a wider choice and (debatably) better quality available.
Are you travelling on a normal singles budget, or wanting more luxury? As a single you'll likely pay through the nose (for accommodation) unless you're careful.
I don't know how much you know about travelling down from CUN, so you may know all this. Cancun City is north of CUN airport and Playa del Carmen is south, and of course Belize is south, so from CUN get a bus (company is probably ADO, though there are others) to Playa, not Cancun. Around US$9. The bus station you arrive at in Playa is the more southerly of the two, on 5th Avenue (which is a pedestrian precinct). The bus station you leave from is the other, more northerly, one, on 20th Avenue. There are a few good moderately priced hotels between the two bus stations.
If you want to come direct to Belize then plan on getting an ADO express bus from Playa to Chetumal leaving at around 9am. Fare is c. US$25, and it takes c. 5 hours. Buses are fast & comfortable but ice cold with A/C, so make sure you have something to keep warm.
From Chetumal you should take the boat (three competing companies) that goes direct to San Pedro (on AC). There are of course alternatives, but this makes by far the most sense. Journey takes around 1.5 hours.
If you decide to stop off on the way the obvious place is Tulum, not too far from Playa. There you can see Mayan ruins and/or go diving, though IMO you can find much better down here in Belize on both counts. Of course you could visit Cozumel before you leave Playa and dive there, but my limited experiences of that haven't been particularly good.
With two weeks I would suggest diving for the first week, then going inland (staying at/near San Ignacio) to do some jungle tours/see some real Mayan ruins. The advantage of moving base isn't just financial - you'll find everything takes so much less time from an inland base. SI in particular is massively cheaper than AC - though of course you CAN pay the earth at an upmarket resort (I wouldn't, but I'm just a skinflint!). Then either return to CUN directly by bus, boat/bus or bus/plane/bus. Note that if you were to visit Tikal in Guatemala ( a short journey from AI) you could proceed to Flores (not far) and from there fly direct to CUN. It costs, but as ever it's a trade-off between time and money. But you'd need to plan out your days carefully for that to make sense.
If you want more detail on any of this just ask.
