if i only had a week, i'd go somewhere close enough that i could maximize my dive/vacation time. i just got back from cozumel. we were there 8 nights and i could have stayed another week and not tired of it. cozumel and playa del carmen are actually very different places, despite being connected by a 40-minute ferry ride. you could spend your playa trip doing cenotes and mainland dives (like tortugas) and visiting ruins ... and you could spend your cozumel trip diving cozumel and exploring the island - in a week you wouldn't be able to dive all the sites, and many of them are so big (and conditions can vary so much) that you can do them more than once (or at a different time of the day) and have a totally different dive.
i've also heard belize is pretty great for diving - i think there's an island off of belize that is supposed to be amazing. a haul to get to, but worth it. don't know the name, though! ditto for cayman - supposed to nice. but people we talked to still thought coz had the most uw life ...