I went from an iPhone to Android and now Windows Phone Mango.
I hated Mango at first, and now I strongly feel it is the most stable/well thought out OS of the bunch. It is missing some of the software support, but it has a lot of Microsoft apps/titles for it and meshes seemlessly with your Xbox if you have one. Including gamer points/achievements, and ability to remotely control your XBOX and such.
However, out of Android / iPhone, I feel Android is more customizable and better for more advanced computer users, you can do a lot with it. But it requires a lot of 'fiddling' to get a balance of performance vs battery life. I've had quite a few crashes, you'll soon learn how to do a hardware reset on the thing. iPhone is much simpler and extremely boring in my opinon, but it makes up for boring with stability and software support. iPhone is to Toyota, as Android is to Audi. (that is---much more sleek, much less stable).