Based on my current level of information, I would say Android. WP7 is waay too young to make any prognosis yet and the iOS is too limited for a long term run. I mean people nowadays like personalization and what do you get since iOS 4? A mere wallpaper... Android however can be modded inside out since Froyo mostly even without root access as far as I know. And not only that, Android is basically on a mass load of devices while for the iOS I'm limited to basically one fixed device with no options. (knowing Apple, iOS 5 on the 3GS will surely lack features...)