Pretty much what i look for is to be able to get my desired rollout with the most effecient gear sizes i can. In stock and 19,88 works well because you dont need to hang such a huge pinion on the motor. It allows you to run slightly smaller pinions and they are still efficient and you get your desired rollout.

In mod, you need to run a 100 because of the rollout needed for mods require too small of a pinion for an 88 spur. It can be accomplished but by the time you get it right, the diameter of the gear has gotten to the point that you do not have efficient gear mesh.