So, I Have been looking into which photo editor to use for app designing and the web too. My only question if I make an app in Photoshop for the phone do I need to re scale it into the iPad size or is it vector like sketch 3? So basically while I have to make a tablet version and phone or just universal.
"If I buy a new car, can I jump sand dunes with it like I can in my motorcycle?"
...... yes you can. Will it jump better? worse? Be more fun? Well, all that depends upon how you drive, which can't be answered here.
Photoshop has a wealth of vector tools and features. If you create vector content within Photoshop.... it's vector content within Photoshop and like all vector content, it can be resized without any real issues.
Photoshop is, generally, one of the most used application for designers. Whether or not it fits your needs and your desires only you can determine. You will find users who prefer Sketch, just as you'll find users who prefer Photoshop, or Illustrator, or.. [insert app name here]
If you create it as a smart object within PS, then resizing isn't going to be a problem. If it is a rasterized object then it it depends on how much or why it needs to be resized. The size is totally up to you , but if you are wanting the smallest possible size for each device then media queries with CSS will accomplish that easy enough if you create a few different sizes. When in doubt create/convert to smart object or keep it as a vector until you know for sure where you are headed with it.
It does depend on how you construct your elements. I'm an advocate for using Illustrator, which is not very typical. But I prefer being able to construct my screens (web, handheld, tablet...) by easily being able to enter actual dimensions for elements and positioning them exactly where I want them, usually b/c I'm respecting some sort of grid. I find that one has to eyeball it more or rely on the info tool panel to get elements positioned exactly. I'd say keep all of your elements vectorized as much as possible to avoid getting into resolution troubles when scaling in your designing and delivering. I recommend Sketch 3 and Illustrator for layout and svg creation, Photoshop for image manipulation and processing.
Sketch is ultimately much faster and quicker to work with. Getting used to the shortcuts may be a pain at first but you never have to worry about not designing in vector based shapes.
I have always been a photoshop/illustrator power user but at my new office they really like Sketch. Sometimes a bit buggy, but definitely much much faster than the alternatives.
Also much more affordable :)