S-Voice is supposed to be a voice assistant for searching the web, making phone calls, etc. It speaks back to you.
Google Now gives you automatic updates on weather, traffic, flights, etc. in the form of cards. It's not always very useful but the things it does is amazing. I was looking up a restaurant online from my pc and when I picked up my phone it gave me traffic conditions and drive time to that restaurant. Pretty cool, but at the same time when I tried to use it for something I actually needed (like a drug store) it gave me results like 45 minutes away (where I had just come from). Location Services drain my battery like crazy so I don't know why I bother keeping it on if it never knows my location.
Google Voice Search allows you to search Google using your voice but that's about it. It is much faster than S Voice for doing this though. Maybe I'm doing something wrong but I cannot make phone calls with Google Now or Voice Search. If I say "Call Billy" it will only bring up a bunch of local businesses. Never really tried texting or opening programs, just assumed it wouldn't work.