The total number of Android app downloads will exceed the number for iOS during 2011, although in value terms Apple will retain its leadership for some time, according to analyst firm Ovum.

More than 18 billion apps will be downloaded in 2011, with 8.1 billion for Android devices and 6 billion for iOS devices. This compares with a total of 7.4 billion apps in 2010, with Android and Apple recording 1.4 billion and 2.7 billion respectively.

Looking further forward, the company expects global downloads to reach 45 billion in 2016. At this point, the total for Android apps will be almost twice those of iOS, with 21.8 billion and 11.6 billion respectively.

In value terms, Ovum said the paid-for apps market will be worth US$3.7 billion in 2011, compared with US$1.95 billion in 2010. This is forecast to rise to US$7.7 billion in 2016, with Android’s share of revenue (US$1.5 billion) at this point still lagging that for Android (US$2.86 billion).

While Android and iOS will dominate in both download and value terms, Ovum forecast that third place will be taken by Windows Phone, overtaking RIM’s BlackBerry.