which one is better

Most linear motors are servo motors, however, most servo motors are rotary motors. What is it exactly that you are asking?

a linear servo motor or a rotary servo motor

jas snipped-for-privacy@yahoo.co.in wrote in news:1146253922.512374.113930 @j33g2000cwa.googlegroups.com:

It depends on what you are after. A linear motor will move a slide faster. A rotary servo will have more torque due to the mechanical advantage of the ball screw.

As far as accuracy, the linear motor would be more accurate in theory due to the lack of mechanical components such as ball screws, belts and couplings. The reality is that linear motors are DC and they generate a lot of heat. That heat is usually under the slide and will cause thermal displacement if it's not dealt with. The heat also robs the motor of power, which it is already lacking.

In terms of protecting the works, a linear motor is more complicated to protect and by it's nature it is more exposed to the environment than a rotary motor. So linear motors are in general more expensive to deploy in a machining environment. The lack of torque also makes it difficult to use linear motors for most heavy machining applications.

Another factor to consider is position feedback. I assume if the OP is looking into servos, then position feedback is needed also (I could be wrong).

Rotary servo motors can have rotary encoders (cheap) easily installed. Linear motors must have linear encoders installed, which usually are more expensive and more difficult to mount.