I'm not finding the link right now, but I remember that in 2010 Zscaler conducted a little test, and Google's Safebrowsing was better than IE's SmartScreen. Of course, this was a 2010 test... we're near 2013. Things change. I believe that Microsoft also gets data to SmartScreen from partners? Now that Google owns Virus Total, and that Virus Total also scans URLs (not just files), we may see some improvement in Safebrowsing... hopefully. Now, which one is better... I never conducted any tests... but, I can say that in my system they're both great.