Well, I don't think I would buy the 15 day pass regardless. If you decide to buy the Swiss Travel Pass, I would just buy an 8 day pass and pay for your full-price tickets for either the first day or last day of your trip, depending on whichever would save you money. You could leverage Saver Day Pass prices for one of the days if available.
The only way to know the best approach is to list out every one of your train rides and use of mountain lifts and then look up the prices for 1/2 fare and see if you save money buy doing that (including the price of the 1/2 price card).
The other pass typically used is the Berner Oberland Pass while in the Wengen area. But since half your time will be in Zermatt, I doubt that pass would be worthwhile, but perhaps if you purchase the 1/2 fare card. There is also a Junfrau Pass which could offer value.
You need to do the math to know which combination will be best for your specific plans.
For my September, the cost difference was not significant. I'm going with the Swiss Travel Pass for convenience. But I'm doing more train trips than you are. Depending on the weather upon arrival in Wengen, I may get the Junfrau Pass if it will save me money. I doubt I'll get a pass in Zermatt. I don't see it adding much value if I have the STP.
