It was a concept that was not so common in the days when there were only round cords, but nowadays many people pursue the sharpness of nylon cords. The basic understanding is that the sharpness increases when the edge of the cord can hit the grass. In terms of durability, a round shape cord is suitable since the burden on it is lower, but I think it is better to start with a thicker sized cord at the beginning stage of use.
On top of this idea, when you considered the twisted shape (twist cord) you could enjoy the better sharpness.
It is the benefit of nylon cord that you could change the sort of cord depending on the kind of brush cutter, environment of the grass/field and the matching with the cutter head etc.
You are kindly advised to try a variety of nylon cord.