Well, most often you can't choose. Often you need the train to get from one city to another and then the bus to get to your final destination that might be a small village. Even though private bus companies try to get into the market, the trains between cities are at the moment still way more frequent and reliable.
In particular for you first two journeys, to Montreaux and Lucerne, I would take the train. As far as I know, there is not even a bus connection available, and even if there would be one, it wouldn't be faster than the direct train that takes 1 hour to Montreaux, respectively three hours to Lucerne, without the need of changing trains.
Check out sbb.ch for up to date time tables. In any case, think about buying a half fare card. You pay 150 CHF and then every ticket that you buy within a year costs only half its original price.