Cavity Brush
I find that a cavity brush (Reedy 769) with the serrations worn away runs very similar to a serrated brush (Reedy 767) still running on the serrations. I get more punch out of the corners with a reduced area brush. I remove the serrations on an old motor and then break them in on the race motor so that the first run will be a good one.
The advantage of the 769 is the surface area in contact with the comm is constant for its lifetime. You can use it until it gets too short.
A 767 will loose most of its serrations and become more like a full face brush after 3-4 runs. Then if you like running on the serrations most guys will replace the brush.