Folks over Pocketnow have posted a video comparison of the three major mobile systems' dictation feature with iPad, Nexus 7, and Lumia 920 runing on iOS 6.0.1,Jelly Bean 4.2, and Windows Phone 8. Note that it's not talking about Siri or S Voice or Google Now here just the stock speech-to-text dictation software you’d use if you were drafting an email or text message by voice.
Siri had a bad presentation in some previous tests, and how is its dictation? As you can see in the video, Nexus 7 and iPad were not good at punctuations, and iOS got the best correct rate. However, iOS needs the network, while Android can do it without network, that is nice.
Take a look: