We are taking the Bernina Exp. route in December. We plan to spend the night and then head on to our next stop which will require us to backtrack. I don't know if we should spend the night in Tirano or go back to Pontresina, St. Moritz or Chur. Any suggestions?
Where are you starting from, and where do you want to go afterward?
Certainly don't stay in Tirano, it is likely to be dreary in December.
When precisely in December? If after Dec 18 or thereabouts, have a look at Bergün. There is a fantastic sledge/toboggan ride there, between the stations of Preda and Bergün. Kurhaus Bergün is a recommended historic hotel.
Before then, I would stick with the St Moritz area, which is more likely to be lively.
I would choose charming Pontresina over St-Moritz