Please suggest through which train we can see beautiful valleys and mountains in switzerland

Both.
They just take different routes.
I only experienced the Bernina Express so far and it was absolutely great.
If you like, book lunch on the train, that is an absolute plus!

Enjoy!

We've done the Glacier Express and really enjoyed it (first class with lunch included), however, it was probably an hour or two too long for our liking. By the end of the trip, we were both thrilled to get out and walk around.

One suggestion, if you do lunch is bring your own wine with you. We brought a very nice bottle and had it with our lunch instead of the not so nice, very high priced bottle available on the train.

Bernina definitely!

Bernina Express in mid-April

Bernina Express: Take it from Chur or St Moritz