Ben Brooks, on Christine Chan’s article comparing the three:
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.
I’ll be sticking with Instapaper.