Hello Kazilla
Swiss citizens have the habit to get on the train at the train’s centre; by doing so, the coaches in the centre seem quickly fully booked while the cars at the back are almost empty. This is also the case in Basel SBB since the escalators are just in the center of the platform, this is where the people get on the train, too. If you go towards the train’s end, however, you will find many empty seats. And, most people get off in Bern, so you can swap seats at Bern station (the train stops there for a few minutes) to a forward-facing seat. And, then, choose a seat on the left-hand side in travel direction (the opposite side from where you can see the platform). Because after the train took a stop at Thun, you will see Lake Thun as well as the Jungfrau mountains on the left-hand side in travel direction. Bern to Interlaken Ost is one of the most picturesque train lines SBB has to offer.
I, personally, would choose the Saver Day Pass; it does cost a few bucks more only yet allows you to use any connection. If you travel on a Supersaver ticket, there is no possibility to upgrade the ticket in case of a delay (e.g. flight delay, river cruise late arrival, etc.), you would have to repurchase a new ticket at standard fare (yes, you can ask for a refund of the Supersaver ticket later, but why the hassle if you can get around it thanks to the Saver Day Pass?)
And yes, as a guest to our country, you are certainly welcome to use the Saver Day Pass. If you hold the Swiss Half Fare Card in addition (you should consider it if you stay longer in Wengen), you can get the Saver Day Pass at a reduced fare. Same for the gondolas and mountain trains in the Jungfrau area.
Please don’t hesitate to ask more questions, we are there to answer them 🙂
Best wishes,
Roland