Whether the Swiss Travel Pass is the best ticket for you depends on your travel and seightseeing plans. It is worth if you are going to use public transportation a lot, or want to use the included museum pass.
Alternatively, the Swiss Half Fare Card might be more beneficial. It entitles you to buy the Saver Day Pass at half fare. And, with the Swiss Half Fare card you do not have to spend ticket costs in advance, you can simply buy the tickets as you go which is usually cheaper.
Just keep in mind that the SDP price is flexible, the higher the demand, the higher the ticket price. It is not in any case cheaper than the Swiss Travel Pass.
The Swiss Travel Pass includes the Museum pass, which gives you access to 500 museums. The Saver Day Pass does not.
The Swiss Travel Pass is not cheaper when bought in advance, unless there is a promotion running.