The Swiss Travel Pass (adult or youth) doesn't offer a lot of convenience if you are going to stay mostly in the same area, as for mountain railways you still need to buy tickets. If you get the Berner Oberland pass you can just bypass the queues at the ticket offices of most cable cars and directly go to the gate, scan your pass and board. Only for the Jungfrau and Schilthorn cable cars would you still have to get a ticket.
I often do advise people to get a pass (either the Jungfrau Pass or the Berner Oberland Pass) and then just forget about what it cost and just try to get a lot of use out of it. With a pass you do things like I did yesterday, going to Gimmelwald just to get a beer...