Sketch is more of a replacement for Fireworks than Photoshop, as in its purely a vector design tool and therefore much better suited to web design. I never really understood why so many people used Photoshop for that task.

If you're looking for a replacement for Photoshop though id recommend Pixelmator.

It's incomparable - in that Sketch is far better prepared for it. Photoshop has never been aimed at that, any the ways people make it happen are sort of 'hacks' they discovered to get to what they need to do.

Sketch has been developed from the ground up as a Screen design tool, and it shows.