Now that Interrail and Eurail are valid on ALMOST all (not all) rail lines, which used not to be the case, I would say the key really, apart from price, is whether you need non-rail validity (e.g. cable cars, ferries and of course Postautos). Here's where IR / ER are valid nowadays:
https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-geltungsbereich-InterRail-EuRail.pdf