What's interesting about Instapaper is that Marco is in a way moving towards embracing both models. With his "3rd-party access only for subscribers" model (see http://blog.instapaper.com/post/3208433429), he can focus on keeping the iOS app polished while letting third-party developers handle writing apps for other platforms. Assuming this actually happens (a big assumption, to be fair), non-iOS platforms will serve as a revenue source by driving subscriptions.