I think buying two passes is a bit of overkill. An 8-day and a 15-day is 703 chf, and what are you going to do the other 4 days? Two 15-day passes is 800 chf. Remember, you will not be traveling from one end of Switzerland to the other every day, some days you will just be taking a local tram to the museum and back, but you are paying about 30 chf everyday for travel. The Half Fare will probably save the most money overall. Buying train tickets out of a vending machine is not too much more difficult than buying a can of soda from a machine. Once you get used to it, its second nature. You select the Half Fare option collect your ticket, validate it and away you go. If the ticket inspector asks, you just show him the ticket and the Half Fare Card. That's the way the Swiss do it, and that's the way you should do it. Unless, of course. money is no object for you. In Switzerland, any way to stretch the budget is usually necessary for those of us mere mortals.
Edit- Rick Steves (and Rail Europe) are running a sale of 20 % off on Swiss Passes for travel completed by the end of June. So you can factor that in as well.