In general, there are more things for non-divers to do in Cancun than in Cozumel, but the diving is generally regarded as much better in Cozumel so you have to weigh it all. You know, what kinds of things does the wife want to do, how understanding is she etc.
Cancun is much more of a sunbather type tourist destination with some good diving. Cozumel is a dive destination with some diversions for your surface intervals. (IMHO)
There are shops catering to the tourists in downtown Cozumel, and there are restaurants and clubs in town. In addition there are some really nice beach clubs in Cozumel, plus you can tour the island. I think there is a small ruin on the island, but I have no personal info to confirm or to rate that.
Remember that if you dive only one 2-tank trip each morning, then you have the afternoon and evening to share with the spousal unit. Of course you know her best (hopefully) and you know what she wants and what she will (and won') allow.
The ferry from Playa Del Carmen runs regularly to Cozumel, costing about $8 and 40 - 50 minutes. You can take a Bus or taxi or shuttle from the airport in Cancun to PDC and that costs about $8 - 15 per person and takes about 45 minutes. I chose that process because it saved me about $150 to fly into Cancun instead, and the flight times worked a lot better for me.
There have been a lot of other posts on SB with more specific details about the ferries, the buses, the diving etc.
I would choose Cozumel over Cancun in a microsecond, but I don't have a wife to consider. You gotta decide whether the better diving is worth it. Of course you could still potentially have a great time in Cancun. Sometimes you have to bring your own good time with you.
Just my opinions tossed into cyberspace, and only because you asked.
Wristshot
There are 10 kinds of people in the world; those that get binary and those that don't.