For a long time, Apple's App Store was the big kahuna. Not only was its inventory vast, but many of the offerings there just weren't available for other platforms. No surprise there. Developers often follow the money, and when it came to sales, the App Store was the undisputed king.

But that tide is starting to shift. Sure, the App Store still boasts more revenue — as of last month, more than 40 billion iOS apps have been downloaded, generating over $7 billion — but Google Play has been steadily closing the gap and emerging as a force to be reckoned with. According to market researcher App Annie, sales of Google Play apps have doubled in Q4 from the previous three months. Meanwhile, Apple's revenue climbed just 20 percent. And this follows the news that Google Play hit a milestone of 700,000 apps in October, the same figure Apple announced in its App Store at the time. (In January, Cupertino updated the number to 800,000+.)

By the end of December, Windows Phone — which usurped BlackBerry as the third-place smartphone platform — only had about 170,000 apps. Before you scoff, it took two years before iOS attracted that many apps. But even so, it's still nowhere near the likes of Google Play or the App Store today. As for BlackBerry, the company announced last month that there are about 70,000 apps available for its new Z10 handset.

When it comes to the top two platforms and the applications in its eco-system, there are a number of things playing into the situation. Issues like fragmentation and frequent software update cycles vexed Android app developers in the past. But things are changing. The platform is maturing and updates aren't as frequent. Plus, Google hired a dedicated pro, Purnima Kochikar, last October to run the store's business development. Her main goal is to ease development and streamline payments. The company is even offering workshops to show devs easier ways to manage apps and boost their userbases. It's paying off. Firms like Ngmoco are starting to put equal emphasis on both platforms, and some are even developing for Android first.

And so the app war is firmly on. The past six months of Google Play's evolution and growth have been breathtaking, and it may not be long before it's ready to take on the almighty App Store head on. There's a lot at stake here. Apps have always been one of the biggest reasons some users choose one platform or device over another, so whichever comes out on top here may end up in the strongest position in the overall mobile market.