Christine Chan does a nice job breaking down all three read-it-later services. What I don’t quite get is this tidbit from her article about Instapaper:

While it does the basics fine, I will admit, the typography and overall design of the app currently does not have the “polish” that everyone sees in Readability.

There’s actually a lot of “polish” in Instapaper. Instapaper just doesn’t have custom fonts, but that doesn’t mean the fonts that it has are bad — they just aren’t new and shiny. Personally I think Readability did a really nice job designing their app, but it’s got a long way to go to match the usability of Instapaper.