I would recommend Acorn. Despite its uncluttered interface it is very powerful. Pixelmator is also very good but the GUI is absolutely horrible, in my opinion. Pallets and huds all over the place and the shiny black makes the tools difficult to find. If it had a proper Mac like UI it would be the one but as it stands, I don't like it.
My tastes are irrelevant though, you may think differently, what we can all agree on, I hope, is that both Acorn and Pixelmator are great apps. One of them will be right for you.