Photoshop is a good program, but in the end its a photo editing program that people can use as an drawing tool, it has a bunch of other bells and whistles that really are not needed for drawing or painting. It simulates painting well enough, but it's just that a simulation.
Recently, I came across a new program by the name of Paint Storm Studio and I must say for painting and art its a lot more natural than any other program out there. The paint mixes naturally and it feels and looks like you are using actual paint.
I'm still playing with it and testing it out. The interface is very clunky, ( the program is still in its infancy) but the variety of brushes and the ability to customize them so much is well worth the $19. That's right its only 19 bucks. It was also created so that it could work on a tablet with touch as well as with a stylus on a desktop or laptop.
I made this digital illustration of Hillary Clinton real quick on a Surface Pro 4 just to see what it could do. Well, I must admit I was not disappointed. The paint mixer alone was pretty bananas! It functioned like the real thing. I even used my fingers a bit not just the stylus.
If you want more information check out their site, they have some good videos and pretty great tutorials.