Is the scenery worth seeing on the Bernina Express between Chur and St. Moritz?

I suppose that's subjective, but I thought the scenery on that route was wonderful.

It may not be for you - I don't know - but for me, most certainly. With a car you can see the general area, but the train goes many places a car can never get.

The Landwasser viaduct and tunnel can be seen from a distance by car or experienced on the train...

To me, that section, known as the Albula Railway, is one of the most scenic of the entire Glacier Express route. At least beyond Thusis. That railway is also a UNESCO world heritage site.
This being said, the road follows the same route and gives better views of the railway viaducts, plus you get the benefit of driving over Albula Pass, which is very pretty.
Therefore, since you have the car, you might as well just drive. If you do drive from Chur, I recommend that you go south via Rte 3 through pretty Lenzerheide, rather than along the railway via Thusis

Thanks all for the replies. I think we'll bypass the train and take our time in a car after all. But glad to get all replies!

I will just add that if you drive, the shortest & fastest route to St. Moritz takes you through Julier Pass, rather than Albula Pass.
Julier Pass is a significantly easier drive, but it is less scenic than Albula Pass, and you will not see the railway at all. Still very beautiful, though. You can decide on the day!