I don't think variety and quality is the biggest issue. I say there is a bigger gap between essential apps between WP vs Android than Android vs iOS. And the quality of those essentials on Android are good enough to use and aren't as crappy as some folks make it out to be. Android Market is far more robust and cheaper than the WP Marketplace. Where iOS kicks Android's ass in apps are games. I would even go far to say Windows Phone have some better made games than Android and are EXCLUSIVE since some are developed from Microsoft themselves. And Apple's iTunes and locking people in an ecosystem has played a huge role for a decade now. Piracy, malware, and hardware fragmentation is what is discouraging developers. Android needs more exclusive apps especially for gaming and a decent pay system without a credit card. The latter is always a prob since the apk will be stored somewhere anyway. Android is very similar to Symbian where you can easily side load apps.
If you want the best apps, the best ecosystem, and like to get paid as a developer, stick to iOS. If you like to mooch (or save money), get 90% of the bare essentials that you can find from iOS, don't mind waiting to get it 2nd, love to side load apps, and don't care for gaming, stick to Android. And if you want the quickest UI without static icons, love the moving live tiles, want some exclusive games which are highly produced apps, and don't mind its marketplace only has 40K right now and still have to pay higher for them sometimes compared to the other two platforms, go with WP. For best apps, iOS is hands down the winner. For more system control and variety in form factors, Android wins. For a fairly quick and intuitive UI, then I gotta say WP beats them both. I'm a multi-platform type of guy, so I never even bother with which one is really better.
