The answer to your question. A laydown brush will give you more RPM. As this brush is "longer" on the comm than a standup brush, it energises each comm segment for longer. This allows each segment to perform work for as long a time period as it can.This generates the maximal amount of rpm.
A standup brush differs because it is "wider" on the comm face. It delivers the biggest hit of current as soon as it gets to each section thus creating more torque. However, because it isn't as long, each segment doesn't get power for quite as long time as the laydown brush. More torque, less rpm.
Hope that helps..
Incidentally, this is the only difference btw a tamiya TZ(stand up) and RZ(Laydown).
Most "Stock" motors use laydown brushes to get as much rpm as they can. Mod motors have rpm a plenty, so they all use stand up brushes to try to gain torque.