I would recomend the Riu Cancun as it has a wide variety of entertainment, room options, food and pools. If you want a real luxury holiday to Cancun I would recommend the "Blanc hotel" - the former being available through First Choice/Thomson, the latter through Thomas Cook. Somewhere inbetwee the two hotels would be the Riu Palace in Cancun- this allows access to the Riu Cancun but the service and hotel is a lot nicer- and not much extra.
Cancun is very lively, a world away from Goa. It is very americanised but also has a wide variety of nice restaurants, entertainment, bars and nightlife.
Christmas is fun in Cancun, I was there myself December. It does lack the festiveness of home or Somewhere cold like a ski holiday or even Goa.
A good idea is a twin centre with maybe Cancun and Riviera Maya, both ways you fly in/from Cancun but Riviera Maya is a lot quieter.
The weather has been a bit unpredictable this winter in Cancun, at 11am I was getting goosebumps from the sea breeze, it should of been touching 30 degree's C instead I would of said between 21-24C, still not chilly, but compared to Goa which was 30+ last week when I was there it is something to think about if you like it HOT!
Before you say it, yes I do get about a bit