They are not at all comparable.
S-Voice is a voice assistant, it will listen to certain commands and respond.
Google Now is much more than that, it can listen to voice but also does much more.
Here are a few links you can look at -
Top 10 Awesome Features of Google Now
How to Use Google Now | The Complete Guide | Digital Trends
Ten Things You Didn’t Know Google Now Could Do | Re/code
it is always seems to alert me about jams and important data while traveling.
