AC Servos have Speed, also have a continuous rated torque, that torque is from zero to full rated RPM, acelleration is also extremely rapid with servos. Servos also have encoders which close the loop between drive and motor.
Big disadvantage with AC Servos is the cost.
If not in a hurry steppers can be used, they do lose torque as speed increases and can lose position if the going gets tough, HOWEVER with properly sized steppers the torque will not be an issue and they wont loose steps because they have been sized as to have enough torque at all forseable conditions.
Stepers are very cheap when compared to AC Servos.
For big machines steppers are not a viable option because you will have to trade far too much speed and acelleration for the sake of accuracy.
Just my opinions based on my stepper Bridgeport CNC and my AC Servo Computurn Lathe.
Hood