There is precious little data on Android units sold. There are estimates but no actual summary. Google themselves don’t seem to know as they don’t report sales, preferring to report “activations.”
Assuming that activations means “unique activations of new devices” then we can draw some equivalency between Google’s reports and the reported units of iOS sold. There is some uncertainty with respect to iPod touch units, but that is not very big (estimates are drawn from Apple’s reporting of overall iOS units at several points in time.)
To estimate Android, I took the download rates currently available and placed them in the month when they were published. I then interpolated the data monthly and quarterly.
To make the comparison more valuable, I chose to index the starting point at the launch of each respective platform and plot quarterly units (or activations). This way we can observe how the platforms ramped in growth and where they were at similar points in their lifetimes.
The following graph results.