Completely depends. Green screen can be good or bad. There’s an app for iOS that includes tracking markers and green screen. Sometimes, I find it best to use white and roto the fingers as that way you have a lot less spill on the fingers. But if the fingers are doing more complex moves, it can complicate the roto. I also like to sometimes composite any reflections back on the screen if it makes sense. That’s more difficult to do with a green screen, and sometimes a black screen is best for that. It really depends on what you need to do and the environment that the phone will be in.