I'll try to make this answer a bit more general than for your specific itinerary.
Quick conclusions
- Many trips on mountain railways/cable cars: Swiss Travel Pass
- Simpler planning and more flexibility: Swiss Travel Pass
- European-domiciled travellers: probably Interrail
- Young travellers (before 25th birthday): probably Swiss Travel Pass
- Frequent travellers: annual half-fare travel card
- Lucky option (if timing is right, if local friends are available): special day pass or municipal day pass
- Standard option: discount day pass
If you will travel a lot on certain touristic mountain railways/cable cars
Certain touristic mountain railways/cable cars such those in Jungfrau region or near Zermatt do not offer discount to the other passes mentioned below, but do have 50% discount for half-fare card holders and Swiss Travel Pass holders.
If you will take these touristic railways/cable cards (e.g. Jungfraujoch or Zermatt-Gornergrat), lack of discount on these very expensive tickets can quickly eat away the otherwise cheaper price of the other passes that are more advantageous for normal train routes.
For example, the savings on Zermatt-Gornergrat alone are worth 63.- (return trip, June-August price). Interrail and other day passes do not give right to discount on this route.
MySwissAlps is a very helpful website when planning trips in Switzerland and it often has detailed information on the discount available to different kind of travel passes.
If you live in Europe and take mostly trains
For European residents, Interrail One Country Pass for Switzerland is a very competitive offer. For example, for three days in one month, it is 49 EUR per day for adults, 42 EUR per day for youths.
Even if you only use two days, it may still be cheaper than other offers.
The youth definition for Interrail is also more extensive, the youth price applies before your 28th birthday (instead of 25th like most Swiss offers).
As it is a rail pass, its covered area is different from the coverage area of Swiss Travel Pass or other Swiss-issued travel passes.
Most train companies (including the main one, Swiss Federal Railways) are included in the pass and some ferries are also discounted.
Urban trains (S-Bahn) are included, but buses and trams are not.
It also gives a discount (often 25%) on certain mountain railways.
Minor differences in coverage area of the other passes exist! For example, the train to Rigi Kulm is included in Swiss-issued passes but only discounted 50% on Interrail.
If you are under 25
Swiss Travel Pass for youth can be a great deal starting at 21 per day (15 days) up to 55.- per day (3 days consecutive) or 63.- per day (3 days flexible). European domicile is not requried.
If you come to Switzerland a lot
You may want to consider getting a half-fare subscription (185.- first year, 165.- afterwards; before 25th birthday: 120.- first year, 100.- afterwards). It gives you 50% discount (or sometimes less on special, regional or low-value fares) on all regular public transport and most mountain railways and cable cars in Switzerland.
You also benefit from a regular day pass at 75.- and further discount on "supersaver" train tickets and day passes.
The regular half-fare subscription does not have a residency requirement. Special offers sometime are only applicable to customers residing in Switzerland and neighbouring countries.
If you will only take a couple train rides in total
Booking specific trains in advance may be cheaper. The SBB website and app are well-made and all Swiss public transport options (train, bus, tram, boat, some mountain railways and cable cards) are integrated. There are often "supersaver" tickets available at a much cheaper price than the normal tariff. Of course, the number of saver tickets is limited and usually the discount becomes smaller as travel dates near. Peak periods (holidays, weekends, etc.) also often have less discount or sell out quickly.
Unlike most normal train tickets (and day passes/Swiss Travel Pass), the tickets are only valid on the specific train at the specific date and time indicated on the tickets.
If you do not make the specific trains on time because your flight got delayed or reasons not of the railways' fault, the tickets are refundable less a fee (10.-) if you purchase a standard ticket in its place.
Urban transport is usually not included in the discount offer. In some cities （e.g. Lucerne) your hotel may offer you a local travel pass (as a benefit for paying the city tax).
If you have friends in Switzerland and your timing is lucky
If you happen to arrive in Switzerland during a period where special, you may benefit from discounted day passes even if you do not have a half-fare card.
Right now (August 2022) for example, Coop/Interdiscount is selling special day passes at 49.- each until September 11, with flexible dates between August 15 and October 2. They only deliver to Switzerland, although you could buy them in person as well (but it will take you time to find a store and the stock is not guaranteed).
Swiss Post and Coop/Interdiscount regularly have special day pass offers, but their availability cannot be reliably predicted. Sometimes, the Swiss Federal Railways itself has special sales.
Many municipalities also offer their residents a limited number of day passes each day for around 40.- to 50.-. These day passes are only usable on the day indicated on the pass and are not refundable even if your flight got delayed or the weather is very bad. Most municipalities require the buyer to be a registered resident (subject to certain exceptions). If you have friends in Switzerland (or better you will be travelling with them), this might be an option to consider.
However, special and municipal day passes often do not give discount on certain mountain railways and cable cars (unlike Swiss Travel Pass).
Discount day passes
If you do not have a half-fare subscription (like most casual tourists), the standard daily option is the one mentioned by the other answer (discounted day pass).
Like special and municipal day passes, saver day passes also usually do not give right to discount on touristic mountain railways and cable cars.
Museums
Swiss Travel Pass also includes free entry to most museums. But most Swiss museums are not especially expensive (e.g. Zurich Art Gallery adult rate is 23.-; National Museum 10.-; ), especially if you are young or a student. If you do not intend to visit more than a couple museums and you can get a travel card at a much cheaper price, the museum benefits are relatively peripheral.
If you intend to visit many museums in one city/region, you could also probably find a local museum pass (sometimes included in a local transport pass, e.g. Zurich Card) for a better deal.
