Photoshop is a powerhouse but has a lot of historical bloat built up over the years particularly around high-resolution bitmap editing. I mean it can paint a car out of a scene with a single brushstroke for chrissakes. Sketch has none of that, but in turn its vector feature set is super streamlined and thoughtful, so it's basically a UI designer's dream.
It would make a little more sense to compare it to Fireworks (may it rest in peace), as that was maybe a bit more suited to UI design than Photoshop.
Other pros
+ Text Styles (Save a font style for h1,h2,h3,p. Why hasn't anybody else done this?)
+ Better color picker because it magnifies to the pixel. Much easier/faster to find colors on text.
+ There's like 10 colors on your active palette. Photoshop is 2.
+ Faster to export images.
+ Duplicate/Copy anything faster than Photoshop.
+ Opens up way faster than Photoshop. Similar to opening a text editor.
+ It's $49 vs $50/month for Adobe CC.
I don't use photoshop anymore. really happy with it