If you were just starting out, I'd suggest you work with Xamarin to do your app development in, and Unity3d if you were doing games or 3d work (Though you could use monogame with xamarin if you were so inclined). But then again, my comfort zone is C#.
Android and iOS apps these days tend to look very similar, with the main difference being a soft back button for iOS and none for Android. Windows phone can look very different if you use the Metro design language but quite a few apps that I've used seem to be mostly ignoring it (admittedly niche ones) and looking like the big two.
Whatever developer you hire will be "better" (or at least more comfortable) at one platform than another.
Have you done your market research? Are you targeting a particular market that is likely to have one phone type rather than another? I've noticed that the different platforms tend to attract different types of people. App Annie is a good source on this market data though you'll have to ignore the most-popular apps as you're not going to be the next facebook/ebay etc no matter how hard you try.
It can be beneficial for both the employer & employee, but it is a bit of a particular case.