Pixelmator and Acorn are both best-in-class image editors for Mac OS X. Developed by small dedicated teams, each app has a long history of updates dating all the way back to September 2007. As far as features are concerned there is very little I can do in Pixelmator that I can't do in Acorn and vice versa. Both apps cost $29.99, (although Pixelmator has received free upgrades since appearing in the App Store). No, the reason I switched from Pixelmator to Acorn has little to do with features and even less to do with price. I switched from Pixelmator to Acorn because I believe in indie Mac app development, and I want to support applications that continue to grow outside of the Mac App Store.
Pixelmator has been an App Store only app since the release of version 2.0 in 2011. At the time I choose Pixelmator as my primary image editor for its impressive features and familiar Photoshop-like user interface. But since October 2011 every Pixelmator update I have received has been free of charge, and over time Pixelmator's progress has slowed. I would argue that since the release of version 3.0 in October 2013, Pixelmator for Mac has not received a single meaningful update. Meanwhile Acorn continues to receive multiple meaningful updates several times a year.
I don't mean to pick on Pixelmator while applauding Acorn, but I believe the difference in their business models is clear. Complex apps like Acorn and Pixelmator cannot thrive indefinitely on a one time fee. Apps require regular paid updates to fuel innovation. Pixelmator is a victim of the myth of the App Store, that somehow indie Mac developers will make up lost revenue in volume by releasing only on the Mac App Store. All while Apple takes their thirty percent off the top. If you value indie Mac development, respect Mac developers by purchasing paid upgrades outside of the Mac App Store when possible. Apple, the wealthiest company in the world doesn't need your thirty percent, and indie Mac developers don't deserve to fall victim to the myth of the App Store.