I'll try to make this as short and easy to read as possible.
First of all, if you go in December you shouldn't worry to much about the wheather, it will be good around the island. The north and west is the best protected area, wich also results better reef life.
The north: Grand Baie and surroundings is the most famous place, it can offer you everything, beautifull beaches, small pubs up to 5 or 6 big clubs. Also for dining this is the best place to get international cuisine.
The west: Flic-en-Flac and surroundings is comparable to Grand Baie, but less lively. It also caters a good nightlife, but just not like Grand Baie. You'll get a bit more natural surroundings here.
The south: Bel Ombre to Le Morne is the quietest place on Mauritius, you'll get beautifull beaches with a fantastic view and acces to the mountains. The resorts Le paradis and Heritage also offer you a golf course. There isn't any nightlife though, and the evenings will be spend at the hotel bars. This is if you want to lay at the beach most of the time, or go mountainbiking through the sugarcane fields, pure relaxing and truely away from the western world.
The east: Flacq to Belle Mare offers you the same as the south, but with more possibilities to go outside your hotel, Centre De Flacq is a nice town, and you'll find some very good local dining near the hotels.
Hope this can help a bit, it's been a while since I was there, but that's how I remember Mauritius.
For me the South and the East where the nicest places to stay. Although if you're sure you want to go out alot for clubbing or dining I would suggest North or West. But to see the true beauty of Mauritius you should visit all