The price delta at some supply places is zero, or slightly higher for xylene. Depends on your source.
Toluene is more available.
Xylene has a very slightly better octane raising effect in theory, and in my turbo.
Xylene, IMHO, is less lubricious - but I don't think it's because of the molecule itself, more because of the refinery method.
I'm not sure which might be harder on fuel system components. Toluene has more trials by BiTOG whackheads, and no one came back screaming.