The harder the screen protector, the less it will scratch - and the easier it will break. The softer it is the more shock it'll absorb (to save the screen) but the easier it will break. So it's a toss-up between plastic, which will scratch but protect the screen, or tempered glass, which is just a bit more scratch resistant than Gorilla Glass (just a bit if it's good tempered glass - some cheap glass protectors aren't as hard as the screen) but will break even if you drop the phone on a corner.