Android phones all day. I had a windows mobile phone and it was awesome flashing different versions of Windows Mobile into a phone. 6.5 was quite customizable but I really disliked how they were pretty much changing the style into the Zune style, I got to try out the Android OS and I really loved the capabilities and the flexibilities the OS has to offer, its great.
In short, Android all day. To hell with the iPhail 4 and the already announced iPhail 5!