Depends (I know, answer to a thousand questions). For painting very small parts, and if the paint is new/fresh, I find I do not need to thin it. Since enamel thinner is quite volatile, some evaporates every time you open the jar, and eventually you will have to add thinner. For large area brush painting, yes, thinning certainly helps reduce brush marks, even for new bottles.
I may be one of the minority in claiming this, but I believe making a good, brush-markless finish with a brush is actually harder than doing a decent airbrush job. The thinning ratio is certainly more critical- with an airbrush you can adjust air pressure and flow rate a bit to compensate for changes in thinner ratio. And- it takes more practice. More so than the amount of practice for airbrushing.
Now, for the bristle type. I prefer natural bristles. I think they tend to be more suble, which seems to lay down a smoother coat. Also, paint thinner (turpentine/mineral spirits), and especially lacquer thinner, react to some synthetic bristle material. Since I primarily use enamels and lacquers, I avoid synthetic bristles to be on safe side.