From the Wall Street Journal: "However, I found the iPhone 4S worked better than the Galaxy Nexus in noisier environments. For instance, in a crowded shopping-mall food court, while neither phone was perfect, the iPhone understood me to say: "I am dictating this email from the very noisy Court at Montgomery Mall on the iPhone"—missing only the word "food" and capitalizing "Court." The Android phone mangled a very similar sentence as: "I am dictating this email on droid phone from the bearing noise for it montgomery mall."
Detailed review by owner of both: "Siri is handy for sending quick messages and looking up basic information, but Dictation is the real winner in my book. I use dictation on the iPhone 4S to write entire articles with enough accuracy that only minimal editing is needed."
Dictation on the iPhone under iOS 5 is shockingly better than dictation on Android ICS on my Google Galaxy Nexus.
I can carry on a complete SMS conversation via voice on iOS without editing, while having to edit every other phrase on ICS.