Android Market is set to challenge Apple’s App Store for global dominance, as app download totals and the number of apps available from the Google portal matches those available from Apple. This means that Apple’s key marketing proposition to the public and developers – that it offers the biggest mobile platform for apps – will become less valid, according to app search provider Xyologic.

Research by the company forecasts that monthly app downloads for Android devices will match those for Apple iOS devices by June 2012, with both app stores expected to register 3.2 billion app downloads.

This projection is based on the hyperexponential growth (growth that is accelerating) of Android Market downloads. And while Xyologix had previously suspected that the pace of growth would slow slightly, this has not yet taken place. Apple’s App Store, meanwhile, is likely to maintain its growth rate rather than accelerate.

The number of monthly downloads provides a good idea of the size of the market according to Xyologic, as figures such as app revenue are harder to find. In September, Apple’s App Store had 1.45 billion app downloads while Android Market had 0.64 billion. However, Android was already outperforming Apple in some markets including the Czech Republic, Poland and Portugal.

In terms of total app downloads during their lifetimes – a figure Apple has made much of – Xyologic thinks Android Market will catch the App Store by May 2013, with a total of 88 billion products download. It is at this point that Xyologic believes Apple will need to change its marketing story.

The other major metric that illustrates app store size is the number of apps available. Xyologic forecasts that Android Market will match the App Store by August 2012, when both are predicted to have around 680,000 apps.

This may have happened by December 2011, had Google not decided to step-up its clearing of low-quality apps from Android Market. As more and more app publishers spam Android Market with app submissions, Google now deletes tens of thousands of apps every month, Xyologic said.

The monthly deletion rate of apps – the number of deleted apps divided by number of new apps – has risen from 20 percent in November 2010 to 48 percent in July 2011. This compares to Apple’s deletion rate of 30 percent in August 2011.