What percentage of Google Play Store apps are available in Windows Phone Store? “relative difference”
As of July, the Google Play Store had approximately 1 million apps (according to Wikipedia). At the end of June, there were 160 thousand apps in the Windows Phone store according to zednet http://www.zdnet.com/microsoft-claims-160000-windows-phone-apps-in-store-drops-dev-costs-7000017381/.
However, one has to take these numbers with a huge grain of salt. These are not *the same* apps, and as a platform develops, it tends to have a larger number of apps competing in a particular category (more "Bubbles" games, for example). If you are asking because you are thinking about buying an Android or Windows phone, you'd do better to go online and have a look at the apps you are likely to want, see what's available and how good they are.