Lane,
You can make your mind up on the day. While you can book the bus ahead, as well as being a long distance bus, it is also a local bus route so will always have room.
The train route is just stupendous. One of the most scenic routes that you can do anywhere. Short of walking the West Highland Way there is no other way to see this scenery as beyond Tulloch it is far from motor roads. Very, very, remote.
You are all but guaranteed to have wildlife sightings.
Corrour is the highest main line station in the UK and one of the remotest, being 10 miles from the nearest motor road, 16 miles from the nearest village. It was built as a private station. Now you can stay there overnight and has a renowned restaurant and is used by walkers.
Between Fort William and Tulloch try to sit on the right hand side of the train for the views of the river gorge. Beyond Tulloch it doesn't matter.
Crianlarich to Oban, although milder countryside is also very scenic. The best is mainly on the left hand side of the train.
There are no advance fares for the route so just turn up on the day and buy your tickets.
I would argue that it is worth it even in bad weather, when you see it in a different light.
But you may validly think that if it's wet just get the journey done.
As long as you accept the extra cost and extra time of the train go for it.
At Crianlarich there is a lovely little station cafe (or was before Covid), as well as facilities in the village.
The bus route also is very worth doing. I am not in anyway belittling that, or ignoring that, It is mainly a coastal route, and is also great scenically, just a different kind of scenery.
