Hi Steve,
I can't speak to the Bernina Express, but I was in Switzerland this past December 6-13. If I had it to do again, I'd move the trip a week later, or put less emphasis on the mountains.
I had a wonderful time, but look carefully at timing in terms of lifts if that is what you want to do. Most of them opened between the 8th and 10th if I remember correctly. A couple didn't open until after I headed home (Grindelwald-First was one) I am sure it changes each year.
I found the Christmas markets in Basel to be my favorite. There were two, and they were busy and full of beautiful things. Great food, great vibe. I stayed there for a couple of days, and took a day trip up to Colmar, France. That market was great too. I didn't make it to Strasbourg, but could have easily--it wasn't much further on the train. I did not like the markets in Luzern at all, and was glad I didn't plan my trip around them. I never made it to Bern, but I heard they have a nice market there.
Once the lifts opened, I had a great time in the Berner Oberland. I had just missed a few days of fresh snow, so it was pretty when I got there. The weather was definitely not ideal though--the only blue sky I saw in a week was up at the higher altitudes. Still, I got a Hiking/sledging pass and spent several days up and down the lifts, watching the skiers and enjoying the views. Mannlichen and Eigergleischer were fantastic.
But down in the valleys, many things like restaurants and shops were still closed. Most had signs saying they open the week before Christmas or so. I had trouble finding a place to stay in Wengen or Murren because so much was closed.
I imagine the Lake Geneva area would also be lovely, especially Montreux and Lausanne.
