Helical gears are generally seen and described as high speed gears as they can take higher loads than equally sized spur gears. Also with a helical gear the two teeth start to engage and gradually increase as the gears rotate this gradual movement makes helical gears operate much more smoothly and quietly that spur gears. Its because of this design that helical gears are used in the majority of car transmissions. 

Reference info:
Marui standard gear set average decelerate rate = 1 : 6.2
Rate < Standard = High Speed (Lower rate for Higher Speed)
Rate > Standard = High Torque (Higher rate for Higher Torque)