The switch simply introduces a couple of parts, ie it connects a resistor and a capacitor.
As spi noted, using an on-off-on results in two settings being the same, namely the middle position (off) and an outer position (on):
ON — connects R8 330r & C9 68n
OFF— no connection
ON — no connection
Or the opposite for the outer positions, but the middle will always be no-connection
I'd go ahead and use the on-off-on.
Down the road after learning more about the circuit and gaining some confidence with it, try spi's idea for a third setting with a second R/C set of values, or not.
ON — connects R8 330r & C9 68n
OFF— no connection
ON — connects sockets for R8a & C9a — try half R8 & C9, try doubling them — go from there in whichever direction is most ear-pleasing
Actually that on-off-on in the stock circuit would make a great producer's-switch feature...
Producer: "It's too avocado, can you make it sound more pomegranate?"
PangeaDestructor: "Sure, how do you like this setting?" flips on-off-on from no-connection to no-connection, strums...
Producer: "Yeah yeah! That second setting is much better. Okay, roll tape, here we go, take 5..."