According to German research group research2guidance, The Android Marketplace could surpass Apple's App Store in size this year. As TechCrunch points out, another analytics company, Distimo, paints a similar picture. The research groups assume that the current growth rates of both markets remain the same. If so, the crossover date will happen sometime this August when both app stores hit 425,000 available apps.
However, the store with the most apps doesn't necessarily translate into the store with the most downloads. A few days ago, iSuppli issued a report stating that Apple will snag 76 percent of the app download market in 2011, and even by 2014, Apple will command 60 percent of the market with Google a moderate second. For anyone who remembers the shareware scene (Mac vs. Windows) in the 90s, it also doesn't mean that "more" means "better."
In the early years of app stores, the number of apps the store had was seen as the most important factor, as it enticed consumers to by hardware for which there were a large number of apps. However, as app stores mature, it's likely that the paradigm will shift, and quality will be seen as the guiding drive behind app store-supported devices. Indeed, Apple already seems to be moving in that direction, removing "sexy apps," eliminating potentially fake reviews and ratings garnered from app promo codes and banning pay-per-install apps. That's not to say quantity isn't important, but once a store has a few hundred thousand apps, the quality of those apps really becomes the deciding factor in what makes one app store superior to the other.