I don't know about Google Now. But Cortana does have way better results than local MS Speech API. My english does suck, so it is very impressive to me that Cortana got me right most of the time.
Maybe I'm not using it right, but Cortana on my Lumia 640 Windows Phone seems to blow Google Now right out of the water in terms of capability and usability except for punctuation in voice recognition.
On the other hand, generally I'm pretty impressed with just the voice recognition/transcription by Google on my Android phones (exception: "ferociously"). Transcriptions in Google Voice on the other hand are, hm, marginally good enough to often get a general gist of a call before I return it, but if I need the actual details of the message there's no choice but to listen to it. This includes calls made by me, from my phone that I also do voice recognition on, into a Google Voice number that I use for some tracking.
It is interesting that the transcriptions in the web interface show how confident they are of the quality for each word by how dark the word is.
