I'm going to build some new 3/8 fuel lines from fuel pump to filter and carb. I've decided to use inverted flare fittings rather than -AN to have a little less of a race car look. I've got both brass and steel inverted flare nuts. Both will obviously work. Brass will look more harmonious with the rest of the brass fittings and adapters and can't rust. Steel is less likely to get rounded off by a wrench. Other than that, are there clear advantages for one or the other?

Well, people will respect you more if you have brass ones. Particularly if they are big.

Well seriously I would go with the brass for corrosion reasons, unless you have stainless.

It's really just a matter of which ever you prefer.

I don't think I've ever seen steel inverted flare nuts rust all that bad myself.