Any comparison of iPhone and Android is bound to generate heat, but I thought David Pogue's smackdown between Siri and Android voice recognition was informative. The bottom line: the iPhone is better for dictation and for voice control of apps. But Android blows Siri away when it comes to web searches.
Siri's dictation gets the edge because it understands a variety of formatting controls, such as "all caps," and all punctuation, such as "dash" and "ellipsis." Android just understands basic punctuation such as "comma," "period," and "exclamation point." Android, however, can do dictation even if you're not online whereas Siri requires an Internet connection. And Android transcribes as you talk, whereas Siri waits until you pause.