Another great thing about Sage is that, like online readers, its state can be synced automatically. Since it stores its feeds in folders of Bookmarks in Firefox, if you use Weave, the state will be synced with no extra configuration.
Since my feeds consist mostly of blogs, web pages, etc. reading in a browser makes the most sense. Integrating with a mail reader doesn't seem as natural a fit fore me.
Also, if you're using Firefox betas, I would assume you're used to disabling compatibility checking. Sage doesn't use any FF chrome, so I imagine it'll continue working just fine with no changes right through FF 6.0.