Score one for Google.
And a big reason for that is China, as Chinese app stores are now a key battleground for the rival operating systems.
In addition to the main Google Play Store, Android users, mostly in China, can also access app stores through internet brands like Tencent and Baidu, and manufacturers like Xiaomi and Huawei. Those third-party Android stores only made about $10 billion in 2016, but will grow to about $36 billion by 2021, App Annie forecasts.
"What a lot of people don't realize is how huge China is," said Danielle Levitas, App Annie's senior vice president of research. "Even if device sales slow, it's the biggest smartphone channel, and that's not going to change."
Just this year, consumer spending on third-party Android apps is expected to double, boosting the platform past Apple, App Annie forecasts.
Across all Android stores, spending is expected to hit about $41 billion this year, just above the Apple App Store's $40 billion, App Annie estimated. Last year, consumers spent about $27 billion across all Android app stores, compared with about $34 billion in Apple's App Stores, the report said.
App Annie's forecasts are based on a model that looks at data from major publishers, economic conditions, conversations with stakeholders and past performance, said Levitas. The report measures consumer spending but doesn't take into account other sources of app revenue, like advertising.