Preface by saying I had to look this up a bit, had an idea in the back of my head from reading stuff around but nothing exact. I'll get the ball rolling anyway. Anyone who sees anything wrong please correct me or expand, as always.
Resolution scaling is a great option if you're looking to eke out an additional 10-15% performance, after tweaking settings. If you're playing at a high resolution like...
From the article.
......Conventional resolution scaling isn’t perfect and does have an image quality impact: you’re rendering fewer pixels. This is particularly noticeable when it comes to distant and high-frequency detail. Sharpening can help, but it’s not a magic bullet.
I believe that FSR/NIS upscale from a lower resolution while applying antialiasing (AA) to smooth any jaggies while also improving the look of textures so they don't look lower resolution. Whereas lowering the render resolution in the game menu ends up with worse looking textures when you upscale it, even if you can apply sharpening filters to smooth any aliasing at the edges.
DLSS is the best solution but only works on Nvidia RTX cards and takes advantage of AI somehow. The other two are different methods of achieving the same thing, that can work on cards without Tensor cores (The AI bit).
So as I understand it for image quality
DLSS>NIS/FSR>In game upscaling+sharpening.
Not sure which is better out of NIS or FSR yet, and of course there are quality/performance levels for each solution that affect the image and frame rate as well.