I'd go with a stiff spring to help keep the brush in contact, resist junk getting in, and better for low speed power likely needed for muddy conditions.

Not convinced brushed motors are much more waterproof. Don't both types have epoxy coated windings? Seems getting rid of the brush & commutator would be a big advantage. There may be an advantage with brushed if it's a bushing type, most all brushless have bearings best as I know. Either way I'd be more concerned with the speed control and receiver.