I've used Instapaper and Pocket heavily over the years. They are both really solid apps, but each excels in its own area. Both apps are widley supported by RSS and Twitter apps. Both are also seeing active development (with the recent sale of Instapaper to Betaworks). I used Instapaper exclusively until about a year ago, when I switched to Pocket. I've recently found myself back on Instapaper, though.
Instapaper provides a dramatically better reading experience and a great selection of fonts. It also has a clear business model. The app is $3.99 and also offers a $1/month add-on for extra features.
Pocket is a better looking app and provides much deeper Evernote integration. As you send articles to Evernote, it actually adds the original URL to the note information. Instapaper just sends the article contents. Pocket is a free app/service, so it has no clear business model at this time.
At the end of the day, the reading experience is what counts. I'd have to say that Instapaper takes the crown. If you want to hear more, check out this episode of the Mac Power Users podcast.