Writing the same sentence with a fountain pen vs ballpoint, it looks much nicer with the fountain pen.

Fountain pens let you vary the width of the line depending on how much force is applied.

Unless you know you have a flexy nib, you shouldn't. Too much force can cause your tines to sproing, and then you need a new nib (or get a nibsmith to fix it, which might be more). Most of my pens have nibs that aren't designed for much line variation.