Google Now is only marginally useful as an assistant, mostly for search and the few Google Services integrated with it. If you have Search/Location History and Location Reporting Turned off, then Google Now is just about useless.
S Voice is better if you actually want to do more with external services (Posting to FB/Twitter, for example) or integrate better with the/more apps on the phone (S Note, etc.).
If S Voice had Siri or Google Now's Speed (or Cortana's Speed), then it would be perfect. Only thing holding it back is the speed to recognize and act on spoken text. Other than that, as an app to interact with the device and its apps, it's superior to Google Now, easily.
Personally, I think S Voice is more comparable to Siri, while Google Now is more comparable to Cortana. I think Google Now and Cortana are somewhat, fundamentally, a bit different than Siri and S Voice. That's just my opinion. I think the rift between the two types of "assistants" will close as time goes on... But Google and Microsoft's leveraging of their Internet Search Assets has led them to taking a bit of a different method of doing things in that field.
Cortana and Google now are a lot more about Predictive Search and working off of collected data from your use of Google/Bing and their various services. Siri and S Voice are more vanilla "virtual assistants" that aim to give a better user experience through deeper device/platform integration - which they do accomplish on their respective devices (Galaxy and iPhone devices).
I think Cortana has a bit of a lead on Google Now because Microsoft develops all the core software for Windows Phone devices, while Google doesn't for most Android devices. Once HTC, LG, Samsung, Sony, etc. replace the Stock Apps (Email, Calendar, Camera, Browser, etc.) it severely limits what Google Now is capable of doing on the device. The OEMs cannot modify Google's "Search" app so there is a wall there and there doesn't seem to be an API surface to properly bridge it - yet.