Wizz is definitely in competition with Sage. In some ways, it even has more features than Sage.
With all of the standard feed formats supported and two Atom formats (0.3 & 1.0), Wizz provides drag and drop functionality, HTTP Authentication, integrated weather information, integrated search functionality, and the ability to share news and email items with others. I haven't seen RSS feeds zipped up yet in my travels on the Internet, so I don't know how an integrated GZip feature would benefit me.
Some of the more interesting features of Wizz is the ability to show or hide read items. This was one of the extensions selling point. After done with each news item, you don't want the news item in your list anymore. Wizz does a great job of managing these feeds.
One thing that Sage has over the Wizz reader is the feed discovery, but the drag and drop is definitely a plus. Another feature is publishing or emailing your feeds. If you have an entire list of feeds, you have the ability to publish your feeds to your web site for the whole world to see.
Anybody podcast? Wizz handles podcasts (enclosures) included with RSS feeds with ease. This is definitely a nice feature to have, since I would expect this feature to be in a fat client RSS application instead of an extension.
The Wizz RSS Reader edges out Sage by a little margin and NewsFox by a lot. There are a lot of features included with this extension. After installing it and testing each RSS Reader, The Wizz RSS Reader definitely comes out ahead.