How should one decide which one is more suitable for a specific task?
Stepper motors have more torque and Servo motors have more speed as far as I know... What about Accuracy and Position control?

 

I don't know how you would decide, you didn't 'specify' the task... The little hobby servos (out of an old R/C plane) I've been playing with lately have an incrediable amount of torque for their size at 5 volts. Much more torque then most of the steppers I have laying around. I've got 3 unipolar steppers, weigh about 15 lbs, 5 volt-4 amp, but haven't got a power supply to test them...

 

I would say the opposite, stepper usually have much less torque than servo's.

As suggested, it depends entirely what you're wanting to use it for - they are COMPLETELY different devices.

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As far as the motors themselves go steppers do have more torque, but servo's are geared, a geared stepper motor can produce an incredible amount of torque. Hobby servo's have feedback though so they know what position they're in and try to maintain it. Steppers don't have feedback so you have to either add it or trust that you're never going to place the stepper under enough load that it will start slipping.

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Per dollar? Per watt? Per unit volume? Per kilogram?

 

Correct you are. This is certainly a topic and question where generalizations do more harm then good. Besides servos and steppers are fundamentally different devices. A comparison of motor types: DC brushed, DC brush-less, AC induction, AC synchronous, etc, etc might be useful but comparing servos to steppers is silly other then to state that they are both electromechanical transducers.

Lefty

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I have to point out that a servo motor is just a motor with positioning feedback, and a stepper with feedback is still technically a servo motor. But let's just call it that since everyone seems to anyways.

I thought one of the advantages of steppers was they did not spark and the only other motors that run off DC (technically it's a really grey area since any motor can run off DC if you have the right inverters and drivers, but you get the idea) and don't spark are brushless motors. So if you needed a non-sparking motor with absolute position control but the electronics for brushless motors did not exist, were not readily available or too expensive, wouldn't that make a stepper your only choice? For either system you'd probably need feedback and gearing for good results.

The only two other advantages I can think of for a stepper motor are that it's much more straightforward to make a stepper hold it's position and that it's simpler to build than a brushed DC motor. Other than that, I personally can't think of what you would use a stepper motor for where you couldn't use another motor so it may just come down to personal preference?

IN my mind a stepper filled in the role of a brushless motor when brushless motors did not exist (simple, sparkless, but driver electronics are more complex than a brushed DC driver).