Since I'm sitting here at the box, I will offer my humble opinion. First of all (my opinion), I've never seen anything as grand, varied and expansive as the Coz reef. The atolls are a bit different than Coz, but very beautiful and primo diving. The sites just outside the reef, just off AC, are a bit less exciting, but also very enjoyable at the same time. Hol Chan is a shallow but fun dive, lots of critters of all types and a lot of structure. Be prepared for just about any type of critter anywhere you dive. I might suggest making at least one trip out to the atolls, or even the Blue Hole. The BH is an experience in itself, but there are a few other dives that trip in fantastic spots. The trip can be rough if the sea is up, but it's all part of the experience.
We bailed out of Cozumel about 6 years ago because of the influx of people. We like our down time to be relaxed. In Jan you will find warm weather (typically 80's daytime, 70's night). You might pack a windbreaker for evenings. You will find a relaxed laid back atmosphere, friendly people, great food and beverages. If you can't relax here, you need professional help.

Been there, done that, the washing machine ate the T-shirt