That also depends on the profession. macOS/iOS usage by scientists, for example, is likely much higher. Anecdotal, but they seem to dominate at conferences I attend.
I see an awful lot of Surface tablets/convertables on conferences and corporate presentations. But they are more in business than in science of creative circles. (and basically they are just fancy laptops)
From what I see I'd say that Apple (laptop+tablet) has taken a dent in the more business oriented world in recent years. Seems no longer to be manager's favorite toy.
On trains, tablets as a formfactor seems to have taken a dive completely. Seem to be replaced by outsized telephones or thin laptops/convertables. (though most of them were used for watching movies anyway, so you wonder if they were used for much custom software at all, or just media consummation devices and VOIP terminals).
Therefore, I'd say for iOS devices I'd go for software sales (and then preferably outside the top so many, don't want to count gigantic but meaningless numbers for Google,facebook,whatsapp c.s. and the more pro games) rather than for device shipments as an estimate for the iOS custom software market.
That also goes for Android of course, but I doubt the figures would be as conclusive anyway :-)