[Arran]

You can use either type for a power line cap, though many prefer to use the x or y rated safety types as they are designed to fail as open rather then shorted, I believe that most of the safety caps are some sort of ceramic cap as the package appears that way. You can probably get away with using ceramics in audio circuits, they used to do that in some later model tube radios in the late 1950s and 60s and I've never noticed a difference, where you can get into trouble with ceramic caps in in the vertical circuits in TV sets, you also can't use them in the HV power supplies in electrostatic TV sets and scopes.
Regards
Arran