I don't know exactly which kind of Dc motor is used in RC servos?
<cynical=on>usually the cheapest DC motor a manufacturer can get away with and not have crappy product reviews or significant amounts of product return due to failures.</cynical>
the two motors you link are physically much larger than a motor you would find in a servo, operate at 12V rather than 6V, and are just
a motor and gearhead. It requires an external DC motor controller and an absolute position feedback device to create a servo-mechanism.
An r/c servo has a motor, a gear reduction, and
a pot for position feedback, and
electronics to control the motor from a simple low frequency PWM signal.