Stainless steel and most other metals cannot scratch glass. If there are any coatings on the windshield it may scratch them. Metal may also be more likely than plastic, due to its greater strength, to press gravel and grit against the glass in a way which may result in scratches.
Jeez, dude. Way to be all smart and on-topic. Here I was just having some Friday morning fun.
Kidding, of course. That's a good point about the coatings and stuff. Though I've had instances where sand or something actually gets "stuck" in a plastic card and then that drags across the windshield.
I should just stay at home if there's frozen stuff outside.