I don't know what would be best for mudding, but I can post up what I do know.
There are two types of motors: brushed and brushless.
I only used brushed, but lots of people use brushless or both, so I will let them post up about brushless.
Brushed motors are defined by the number of turns or winds on the motor. Commonly referred to as T. The less turns on the motor the faster it is. Example: an 8T motor is faster then a 15T motor. Stock silver cans that come in kits (usually with no labels or markings) are 27T. Most builders of crawlers and trail trucks use a motor between 35T and 75T. I usually use between 35T and 55T.