les 3 vallees

Personally I would avoid Mottaret as the slopes going into it are always busy with beginners and often icy. In December, VT is probably your best bet. Courcheval too expensive, thanks to the Russians unless you base yourself at the lower villages, but the snow may not be there in any quantity.
From VT you can always ski down to Les Menuires and ski lower slopes if the wind is too strong at the top

Looking at your dates - Most self catering accommodation in the area is let on a Saturday through Saturday basis, so you may find it tricky to do two weeks across three letting weeks. Also, New Year will be busy, so book early if you can.
Good luck!