Arguably WP is better than Android, but I watched my L920 become more and more of a slug and perform worse with less battery life with each upgrade. Last year's big upgrade broke the phone and I had to do a hard reset. What finally has made me switch to Android is the lack of interest by M$ in their user base. I loathe the no settings approach to software which makes any chance of customisation or improving performance impossible. But, the XBox music app may well be insanely badly thought out on Android (you can only download albums offline if u put them in a playlist, wtf?), but at least it doesn't take the phone down with it. Hell, every time M$ 'upgrades' this pile of rubbish, the UI gets worse and the product more cumbersome to use. Winamp in 1999 could handle things better, even Zune could and this is 2015. Final straw for me was new versions of Office and other apps going to iOS and Android before they get to a loyal user base and being stuck with IE which just won't load a secure page rendering ebanking, Stripe, WHM/cPanel and more useless with no extra app available.
I'm happy on Android, it is boring, Droid Sans is a horrible font and so on but I can do Firefox with Adblock and access almost anything in browser or app needed to run my business. I can also upgrade without tethering getting broken for no clear reason. Hoping the cumbersome Outlook app handles contacts and calendar better real soon, but like everything from M$, it could take a while.