Despite having larger sale numbers the Android Market has always lagged behind iOS in terms of total app downloads. It seems that that may be coming to an end in the near future says Business Insider:

Google also announced it is now at a billion downloads per month. Apple hit that mark in July of this year. So both stores hit that magic number about three years after launch. And both are now growing at around the same rate.

Check out the above chart to se it for yourself.

As you can see iOS growth is faster but due to the sheer number of Android devices, it will inevitably catch up and surpass it.