Apple claims it can migrate contacts, mail, messages, calendars, and camera roll easily using its Move to iOS app for Android. Of course, you will need to buy all new apps – your Android ones will not work, and your MP3 music collection probably won’t play.
Google has fought back saying that it can make it even easier with its updated iOS to Android switch – that does everything Move to iOS does in reverse, and also brings over your iTunes Music to Google Play.
It will also move your iCloud contents to Google Drive. It is not limited to Apple’s mail/contacts/calendar and has support for many popular apps like Outlook. However if you are using that on iOS then it will have everything in the Exchange 365 cloud.
If you are contemplating purchasing an iPhone or Android device over Christmas, be assured that you can swap from one to the other. For both, purchased apps, and for iOS music, are the casualties of this war.