Testing on devices is quicker than simulators and emulators, though there is more setup and connectivity involved for android devices (installing the device drivers from the device manufacturer on your computer, so your computer can talk to your device). I have certified software at a few companies, all of which have used real devices, though we often used emulators and simulators for developing test automation code. It is advisable to start with the most popular versions of devices, for example, the latest Samsung Galaxy phone and the latest Apple phone, unless you know your audience uses other devices because of a niche market sector that you're tapping into. Simulators and emulators are more convenient, so you might want to develop test automation against them but then make sure that the code works against at least one device before checkin. I have always found it useful to use my own phones for testing because I have lots of apps installed on them, which presents a better test bed for finding bugs but a dedicated company-sponsored phone that isn't someone's personal phone is more desirable for certification purposes.