5 years from now most of the currently installed PCs will be gone, and many of them will be replaced by tablets. Will those tablets be running iOS, Android, or Windows?
It doesn't matter if the tablets are running iOS, Android, or Windows. What does and will matter is, which can run Excel, which can run Word, and which can run Eclipse or Visual Studio? Because, besides IE6 apps that will continue to slowly fade, that's what businesses actually care about.
That's an open question. I would ask instead which ecosystem - Google, Bing/Hotmail/SkyDrive/Azure, Apple, Yahoo, etc. the customer uses as their "home" ecosystem.