Your question about whether it is worth spending Christmas Eve in Paris would be better asked in the Paris forum. Shops in Brugge will of course be closed on Friday 25 December and Sunday 27th, but open on the Saturday. The snow and ice sculpture festival will still be on (next to the railway station) and unless you particularly want to visit a specific museum or other "sight" (e.g. Choco-Story is closed for the whole of Christmas), what makes Brugge interesting is in my opinion not so much individual buildings you could visit, as the city itself, which you can see by just walking around.