Italian trains are pretty cheap, and can be cheaper if booked in advance, but you'll also be using them on longer distances than most people, so the two-country Select Pass could still be worth comparing. Because the Select Pass offers a group discount and the Swiss Travel Pass doesn't, you can get 6 travel days in both countries for $333 per person, versus 4 travel days within a month in just Switzerland for $327.
You have to consider costs for the totality of your plans to really choose the best fitting option for the number of days you'd use trains, as well as lifts or other local transport that might be either covered or discounted in Switzerland. The Swiss Travel Pass covers more, but only on your counted days of pass use, so you might end up buying more days. See more coverage comparisons for Switzerland. All three named scenic trains are covered the same by either type of pass, with the seat reservations costing extra.
EasyJet also has some cheap flights nonstop from Naples to Swiss airports, but usually early a.m. and not operating daily.