We're thinking of going to one of the 2 in January. Opinios please. TIA.

Have been to both, and though there are some beautiful resorts in Cabo, the actual town isn't very nice, and the waters are generally too rough to be swimmable.
Cancun has some of the most beautiful water in the world, as well as fantastic beaches.
Great hotels, too! And restaurants!

If you want gorgeous white sand beach and swimmable turquoise ocean - Cancun.

Cancun hands down! Especially in January. Cabo beaches are no comparison for those in Cancun. We're going to be there in January also.

First question, how old are you and what are you looking to do there? If you want to party, bar and club hop and deal with spring breakers - then yes Cancun is beautiful and very americanized. Cabo is for couples, and families and as I've said on this site before you get what you pay for. So those who are anti cabo probably stayed for cheap in a dumpy hotel. Cabo's water is incredible and the food is much better then in Cancun b/c they are appealing to an older/pickier crowd.

The Yucatan has much better snorkeling as well as interesting historical sites. However, do not stay in Cancun. Playa del Carmen is much nicer and only about 30 miles south of the Cancun airport. From PDC you can easily take the ferry to Cozumel or drive to Tulum.

saying Cabo's water is incredible...that's the first time I've heard that...most beaches are not very swimmable. Good fishing, yes, good swimming/snorkeling, no.
There are also excellent and high-end family resorts and restaurants in Cancun.