Scenic train Swiss and Austra Alps

Basically because there is none. Take a look at the Swiss railway map.
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelsystem.pdf?sfvrsn=7788603a_36

You could take the train to Scuol-Tarasp, then the postbus to Landeck-Zams where you can pick up the Railjet to Salzburg.

You can add the Bernina Express - St Moritz southwest to Tirano and Lugano. Then head east through Como to Verona/Bolzano/Innsbruck/Salzburg. Another approach would be to go to Lucerne and take the Gotthard Panorama Express south to Bellinzona/Lugano (Lake Lucerne steamer and then the old, windy, over the Gotthard pass train. That gets you to the neighborhood of Lake Como. From Como head east to Verona, north to Bolzano and back through the Brenner Pass / Dolomite Alps to Innsbruck and from there on to Salzburg.

https://www.sbb.ch/en/freizeit-ferien/reisen-schweiz/panoramareisen/bernina-express.html

Is one of the two ways a better choice and/or less costly than another?

Both routes mentioned in the previous post are going away from the direction of Salzburg, so will take a long time and cost a lot. The most direct route is to take the train back to Chur, then to Sargans, then you can get the Railjet to Salzburg. There are plenty of scenic mountains on the way. Both the Glacier Express and Bernina Express cover the route from St Moritz to Chur, but you have already covered it on the Glacier Express. So no need to pay extra for the reservation fees on these tourist trains, just take one of the regular hourly trains.

then to Sargans, then you can get the Railjet to Salzburg.

And the most scenic variant of that leg would be to leave the RJ (which takes the route via Rosenheim in Bavaria) in Innsbruck and to switch to a regional train via Zell am See and Schwarzach-St. Veit. So you could see the Kitzbüheler Alpen, the Kaisergebirge, the Loferer Steinberge and the Steinernes Meer in sequence.