I think it's more clear to me now.

Requirements
Speed: standard 0-2000mm/min max 3000mm/min
RPM needed:
5mm lead; standard 0-400RPM, max 600RPM
10mm lead; standard 0-200RPM, max 300RPM

A stepper motor seems to lose it torque from 300-500RPM
A standard servo motor has full torque up to 3000RPM. However to utilize the whole range I would need a 10:1 or a 5:1 reduction.

5mm lead will put me over the speed where the stepper starts to loose torque but it will also give me twice the force of a 10mm lead. I'm not sure i'm going to machine over 2000mm/min but for rapids I need a higher speed.

The weight of the X and Y cart is 30Kg. The Z cart is 20Kg
Cutting force; max 150N

After reading more on servos the torque advantages are clear. If my machine was bigger it would be obvious to use them, with the size om my machine there is not so much of a difference.