Walt Mossberg reviews and compares voice dictation with Android (on a Samsung Galaxy Nexus) and iOS (on an iPhone 4S):
I found that both platforms’ dictation systems worked well enough for me to recommend them. In case after case, both phones got it right, or close enough to require little correcting.
I can only speak for the iPhone 4S and iPad (3rd generation), but I too have been very impressed with voice dictation results when I’ve used it. Though, my results in noisier environments haven’t been as good as Walt’s seemed to be.