Pixelmator is a bitmap graphics program (like Photoshop and Acorn), while Sketch is a vector graphics program (like Illustrator and Inkscape). Both have different uses.

I used Fireworks for half my career, and when it became obvious that Adobe was letting Fw rot, I went looking for alternatives. It took a little while, but Sketch has finally gotten there. Sketch a purely vector app, whereas Fireworks was a strange but workable hybrid of bitmap and vector. When I need to throw down some raster graphics, I fire up Pixelmator.

I too find Fireworks CS4 too useful to dump entirely. Particularly canvas resizing.

I also like Sketch -- it's a very powerful app and very polished, though I find I use it a lot less than I thought I would since it seems to occupy an uneasy place between OmniGraffle and Acorn in my workflow. I wouldn't be surprised if I use it a lot more after another few versions...

I like Sketch, but it has really poor performance on my MacBook Air. A file of comparable size is way smoother in Illustrator than it is in Sketch from my experience.