I prefer the Bernina express, although my habit is to take the regular train over the same path and save some money. If they are still running the open cars in the summer, that would be worth the extra money. If you are spending any time in Chur, consider a train up and back to Alioso (spelling not warranted). That is a really good ride.

Its been a few years, but I seem to recall that the Bernina express is in part seasonal with bus service part of the way over another route during the winter.

Try to keep some flexibility in your schedule. A scenic alpine train ride is a huge disappointment on a hazy, foggy, raining, cloudy day. A bad visibility day isn’t all that rare and it can be perfectly clear at the start and visibility zero as you get higher. Most of the departure stations for the scenic trains have visibility/weather info for the day available.