Spigen NeoFlex Film 2-Pack ($9)
Another option for Galaxy S9 protection is the Spigen NeoFlex film. This isn't a tempered glass screen protector, but a durable film, and that's fine. If you've had trouble with tempered glass on curved screens, you probably don't trust too many. If that's the case, you can't go wrong with something from Spigen.
Spigen's NeoFlex film is a military-grade 3M film that's extremely durable. They recommend installing it with the case already on your phone. That way it fits perfectly. This film won't have any weird textures, no rainbow effects, and won't discolor or yellow over time. It's your best bet if you use a case.
The wet installation method ensures you won't have any bubbles on the screen once you're done either, so that's a plus. Oh, and if you run into problems you get two, so you can try again. Otherwise, give the second protector to a friend, family member, or save it as a backup.
Additionally, buy a Spigen case with a 3D Tempered Glass screen protector built-in right here.