iOS 9 supports a lot of devices that run the system differently, each device has its own specs and its own power so it actually doesn't run the same on every device, if you have a little old device you would be confused whether to update or not, trying the software is the best method to know whether your device is capable of running the OS perfectly or not, personally I did a very big mistake by upgrading to iOS 8, and It was too late to downgrade as apple stops signing its old software after the the new update is released. So I was stuck with iOS 8, now the turn is on iOS 9, apple gave developers and non developers the ability to try iOS 9 before it is released, first, to identify bugs (as a developer), second, to know whether your device would have any problems running it. So, before you update just try the public beta first.
