Wirecutter tested the lot and found that cheapo screen protectors are just as good as name-brand ones. It picks the $13-for-two "Spigen" offering but also lists the "amFilm" one that I have and can vouch for (it's the "Amazon Choice"). They're all the same; Spigen just provides the nicest applicator.
We skipped testing glass from well-known brands, including Belkin, BodyGuardz, and Zagg (Zagg also owns the InvisibleShield line). These protectors are widely available, but they're almost always expensive, and we've found their glass to be of the same quality at best, and sometimes worse. More expensive protectors offer no benefits over the cheaper ones we recommend.
My screen may be unscratched, but the back is well-cracked. Lesson learned: thin hard plastic cases are themselves only good against surface scratches and offer little drop protection.