If can be answered at one glance

Almost always, the one month Half Fare Card for 120 chf is a winner. Swiss Passes seem a bit over priced to me. You can go to www.sbb.ch and add up all the prices individually, remember the first price that shows up on the train list is the half fare price. With the Half Fare Card, you still need to buy your tickets every day, whereas with a Pass you just hop on the train, except for the Jungfraujoch, which is with 25% discount. Surprisingly, with the Half Fare Card, you get the whole 50% discount.

The "problems" are that the Swiss offer so many options and that passes covers so many services that you have to add up. Aside from that Half-Fare Card, the most likely option for a 10-day stay are the Swiss pass for 15 consecutive days (about $485), which fully covers most transportation (solid lines on the Swiss Pass Map ) and museums, but still only gives discounts on higher mountain lifts. Pre-paying allows you the flexibility to just hop on the covered transportation without stopping to buy tickets.

In between those two extremes, there are cheaper flexi passes offering full coverage on a few days and 50% discounts on other days, requiring more planning and calculation to use them wisely. There's also an 8-day consecutive version, if it fits your final trip plan.

Thanks to Sam and Laura. It seems I have to decide.
One last question is about buying them. Should I buy them online (which link?) or from the Zurich station as I arrive?

Assuming you are arriving at Zurich airport, you can buy them at the train station there.

Kemal,

I've found that trying to decide on the best pass in Switzerland is just about impossible, as the correct answer sometimes depends on which specific locations will be visited. I spent a lot of effort trying to work that out prior to my last visit there, including use of a spreadsheet and finally just made an "educated guess" at the best one to buy.

When is your trip taking place? Note that some of the regional passes such as the Berner Oberland pass and Jungfrau VIP pass are typically only available between May and October.

Ken, it will be end of august for 10 days. I already mentioned the plan. Could I have your educated guess please if possible.

I am confused about your reference to Colmar. Colmar is in France (Alsace to be precise).
"Interlaken . . . . - maybe train to Colmar then Zurich" involves a Swiss train Interlaken to Basel, then a French train to Colmar. And on the return leg, a French train Colmar to Basel then a Swiss train to Züich. Your Swiss pass would not cover you Basel to Colmar or Colmar to Basel. You would have to buy separate SNCF (French railways) tickets for this.

Sorry about the confusion. To simplify, let us skip Colmar leg, assuming that if I visit Colmar, I have to accept to pay extra.

Colmar is just a 45 minute ride from Basel and trains run every half hour. 13.70 euro each way, but travel between 8:00 and 17:00 or after 19:00 and if 60 or older, 25% discount.

Kemal,

Without doing a LOT of number crunching for each of the trips you plan on making (which is what I did for my trip), I'd only be able to make a very rough guess.

Your choices will be the Swiss Half Fare Card, Swiss Card, Swiss Flexipass or Swiss Pass. I don't think any of the Regional passes will fit your situation, so I didn't check them. As I recall, some of the Passes can only be purchased outside of Switzerland, so you may be limited in what types you'll be able to use.

You could buy a Swiss Half Fare Card at the first station you stop at, which will provide a 50% discount on most of the routes you'll be using. THIS website provides the details. That may be the most realistic solution, given the short time until your trip.

Hopefully Laura or one of the others will be able to chime in on this question also.

We stayed in Interlaken in 2004 before we had a RS book. We stayed in Muerren last month and there is simply no comparison if the mountains are what you want to experience. It is relatively easy to get to Muerren from Interlaken. It is totally worth the extra effort. Interlaken is near the Alps. Muerren is in the Alps. Or Gimmelwald or Wengen. Lauterbrunnen doesn't get up into the mountains but it is a lot closer than Interlaken and only a 20 minute train ride from Interlaken. Muerren is only another 20-30 minutes by cable car and train or by bus and cable car. The transfers are easy. My wife and I are in our 60s and had no trouble. The rail pass covers the routes to all of these places I mentioned and reduces fares to other places up in the mountains. Even better, the pass covers these routes without adding any days so you still have all of your days on the train before and after your stay in the BO (Berner Oberland).