Historically, iOS has always topped Android in terms of app revenue. There are several reasons for this - the vast majority of Android devices are cheap phones (owners of low-end phones aren't as likely to pay a premium for apps), piracy is much easier, etc. But according to a report from App Annie, that could finally change this year.
The report compares the revenue of both the Play Store and third-party app stores to Apple's App Store. It is estimated that Google Play will generate $42 billion, third-party stores will earn $36 billion, and the iOS App Store will generate $60 billion. That's a lot of in-app purchases.
The third-party app stores include marketplaces from Xiaomi, Huawei, Baidu, Tencent, and others. China, U.S., Japan, South Korea, and the U.K were the top countries by revenue in 2016, and together accounted for 75% of app store revenue (across all platforms and stores) in 2016.