To anyone who thinks that iPhone has revolutionized Speech recognition on their Apple iPhone 4S with Siri - They probably don't know that Windows Phone Mango already has an inbuilt speech recognition assistant.Check out the following video where WinRumors will demonstrate the features of the personal assistant on Windows Phone.
Given the option of choosing between an expensive iPhone or a reasonably prices Windows Phone, my vote goes to Windows Phone on this one.
source - WinRumors