Hello @elianafranzenburg!
Having just gone “lens shopping,” I’ve heard a lot of good things about the 85mm 1.8 from a buddy of mine. He says it’s an extremely sharp lens, and it’s his go-to lens for portraits. Also just having received my first prime (the Canon EF 50mm 1.8), I can agree with @josiahw in that it is nearly impossible to get desired focus at 1.8 (the corner of your subject’s eye will be in focus, and the nose, ear, and the other eye won’t be—sometime’s it’s what you want, and it’s nice to have the capability, but I don’t think it’s what I want most of the time). However, do note that most lenses may not be sharpest at their widest aperture, so if you have a f/2.8 lens, it might end up being sharpest at f/4, with a f/4 lens being sharpest at f/5.6—but it honestly depends on the specific lens. I’m not sure how well a prime lens would handle nature photography, but I guess it depends on what you are trying to photograph.
If your are trying to consider which one to go with, I would highly recommend checking out this site: https://www.the-digital-picture.com
It’s full of very in-depth reviews of just about every lens you could think of, but I think it would be best for you to start with his “recommendations” tab to find the lens that works best for you (perhaps you’ll find you need to save a bit more).