Paint.net by itself is not very good, you need to download some plugins and other tools. But it's easier to use and faster to do simple things. Photoshop requires a lot more study to be used. But it is a complete and professional tool.
So ...
I use paint.net generally for quick things (memes or montages) and photoshop for something more technical (logo, gifs and stuff like that) (I used google translate)