Bernina Express tickets type

Via Samedan-Pontresina is the correct one!

What date are you looking at?
You want the 8:32 departure from Chur. Train 951.

Date that I am looking at is first June. It’s a tentative date for now.

Looks like the 8:32 departure is sold out on June 1.
There is a 13:33 departure that is available. arrives Tirano at 17:59. 63 CHF 2nd cl, 111 CH 1st cl. Plus 26 CH seat reservation fee each.
Look here: https://shop.rhb.ch/en/bernina-express

The regular Swiss trains travel over the exact same train tracks on the same route from Chur to Tirano.
So, if you don’t absolutely, positively want a train with the panoramic windows overhead that the “Bernina Express” trains have (photographers complain about reflections appearing in their overhead photos) you can book a regular train for less.
Have a great Trip!

Despite the name, Bernina Express is not non-stop. It stops at several places between Chur and Tirano. The stop at Alp Grum (or was it Ospizia Bernina, anyway the one way up high in the pass) was about 15 minutes - long enough to get out for a short walk.

The Bernina express, just like most Swiss trains, cannot be sold out.

When you buy a ticket you do not actually buy a "train ticket", you buy a transportation ticket, that permits you to travel from one point to another. And where there are more options you are offered to buy one of those. But usually the SBB site will give you the most logical one first. Just pick that. Even if you were to take the wrong route the conductor will if necessary modify your ticket.

This ticket is then valid for any train on that route, so you can take any train from Chur to Tirano via Samedan and Pontresina.
The Bernina Express train itself carries panoramic coaches, in for those you need a reservation and a surcharge. And yes, all the reserved places can be booked. But the train has three normal carriages in the front where you can sit without that reservation, so you can always travel.

Also the normal carriages have windows that can be opened. Normaly the windows are quite clean but if you can put your camera out of an opened window may result in nicer photos. Look for these windows at close to the doors.

Also in summer there sre several trains that have summer carriages added wich are completely open air.

and make sure to watch youtube videos first, the site is full of them.

As noted you can stop along the way, but a tourist likely wouldn't want to