Frank, a synchronous motor will only operate (rotate) at their designated frequency, this cannot be altered, unless you can alter the frequency (have a look on the net for
drive corrector or variable frequency drives (VFDs)).
Steppers and Servo motors...is an old joust. They both have properties that make them good for use and each have properties that make them better than each other in respective areas.
Steppers: pulses can be counted thus easy to track how many pulses sent and thus where the system is pointing. Steppers have more poles and can make very small steps. Steppers suffer at high speed from torque loss. Steppers are no fuss long life systems.
Servo motors (DC motors), Good torque useful for low and high speed applications, require an encoder to tell how much rotation.
Reality is that despite what motor system is chosen you will need some control of it to correct minor errors. Have a look on the net and these sites for
Bartels drive and
roboscope, even
our own site has drive info for dobs.