Apple currently has over 550,000 apps in their App Store whereas the Android market has only 400,000 apps. Developers prefer releasing apps on Apple's platform because Android platform vary from phone to phone which costs developers more money to adjust to each Android smart phone. The testing and releasing process for a app in the App Store is much easier for developers. Top apps from the App Store tend to be games while Android apps are more business related which is also why Android apps are on average 2.5 times more expensive than Apple's. Apple's secure management of their App Store is also their downfall because it takes a lot for apps to get processed and become available for download. On the other hand, Android's open market approach has resulted in malware problems in their Android Market.
Apple App Store vs Android Market