Christian Zibreg, writing for iDownloadBlog:

Various editions of iOS 9 are currently installed on 87 percent of iPhone, iPad and iPod touch devices, a one-point gain versus a month ago.

And:

By comparison, Google’s official stats have Android 6.0 Marshmallow powering 15.2 percent of smartphones and tablets, with the two-year-old Lollipop (5.0-5.1) OS found on more than one-third of Android hardware (35.3 percent).

Here’s a link to Apple’s official dashboard that measures adoption.