Because of the various updates (i.e. heal tool) promised in Pixelmator 2 I bought Pixelmator 1.whatever and started using it as my go to editor.
As of January 2012 however, there is a weird bug that does not let me arbitrarily set the width and height pixels when cropping an image. Strangely both programs have a bug in this tool although they have different symptoms. My version of Acorn is behind since the Acorn 3 update is another $20 (I thought it was $50, but the author corrected me), so it may not have this issue, but I’m stuck using other programs to work around this bug which is annoying (we’ll see if it is $20 annoying soon).
I can’t speak to Acorn 3 vs Pixelmator 2, but in Acorn 2 vs Pixelmator 2, the latter wins handily.