Here are two useful links on the breakdown of actual mobile users for each of Android and iPhone:
The reason I love these stats is that: a) they are meaningful, and b) they are meaningful. They are first meaningful because they record active users (people who access the Google PlayStore, and the Apple AppStore, respectively), as opposed to based on sales of handsets, or stats of some website that is focused on a particular group (e.g. Apple developers!!) And they are meaningful because almost all active users will have visited these particular sites at least once a month, the survey times are 14 days, so we're talking a boat-load of people. Big numbers means reliable stats (all else being equal - see previous comment about the likely lack of bias).
The big disappointment is both Google and Apple choose to show percentages, not actual numbers. And no breakdown by country of the user. (If you know how to get those stats, please let me know!!!)
I saw a thread on Slashdot saying that these stats show that Android handset market is much more fragmented than Apple. It does in fact show that 94% of people who visit the AppStore have upgraded to iOS6. There is some bias there, as I think they might be strongly encouraged to when they visit the AppStore? (Just checked, and it constantly reminds you to update, even if you don't visit AppStore). But, anyway, if the above theory that practically all active users visit the AppStore at least once a month is true, then it is useful information for a developer (i.e. testing on iOS6 should be sufficient).
Android users, on the other hand, get upgraded by their network provider? We still need to test on each of Android 2.3, 4.0 and 4.1.
But even more important for a developer, if you want to do any animation or special effects, is knowing the hardware power of the devices. The Android 2.3 stats suggest that percentage of users are still on relatively old hardware, single core. And that the other 2/3rds of users probably have a bit more firepower. For Apple users I've no idea of the breakdown. (Again, information donations are very welcome!)