plumbing question - PEX fittings; brass vs plastic

You've got your terminologies mixed. PEX = cross linked polyethylene and is a name for a type of that pipe. All types of fittings can be attached to PEX pipe using crimp rings or some of the newer self sealing push-fit fittings like Sharkbite, Blue Hawk, etc.

I prefer to use metal fittings, when possible. They deform less and don't break as easily as plastic.