Of the two, I've only been to Cozumel - several times. Great drift diving in the park, fast drift diving on the northern reefs, safer than most American cities, friendly locals, good food, and more. You can get a lot of info on how to Do-Cozumel on that forum here.
September is the middle of hurricane season. Hits are rare and I usually go in last August, only once evacuating ahead of a storm. I should have canceled before departure as it was forecast to hit, but I hoped it would turn - then when it didn't, escaped before it landed.
I don't know where you are coming from, but Europeans often say they prefer flying into Cancun rather than Coz as they can get to Cancun non-stop and avoid US I&C and TSA when changing planes on the way to Coz. It's a pity we don't have better international terminals in the US. If you fly into Cancun, it's rather easy to get to Coz - bus, shared van, private van or taxi & ferry, or the small plane service. Lots of info on that on the Coz forum.
