Hello:

Will be adding a gang box for a new outlet, and also one for a wall switch
in a home residence.

Guess I'm really old fashioned, but all the ones I've installed were always
metal.
The green ground wire was always grounded to the metal box, of course.

I see now that they sell plastic ones, as well as what I like to call the
conventional
metal kinds.

My questions a

Are the plastic "legal" per building codes these days ? Allover ?

Or, does it depend on the local state building code (I live in Mass.) ?

Which are presently more popular ?

Thanks,
Bob