I think servos offer better control and more consistent torque. I suppose an oversize stepper would be just as reliable as "the right size servo".
I haven't researched all the diff, but I did build a little device using a stepper drive once, and I wasn't all that happy with the stall characteristics of the stepper I was using. That system had no feedback loop.
A modern servo controller has software settings for integral, differential and proportional gain. I do not know if a stepper drive can simulate this or not (someone who knows can pitch in here ) Fine tuning these settings allows for extreme control of the motor as it accelerates and decelerates, to get rid of jerk, and overshoot, and yet gives good holding torque and speedy response to positioning errors ( as work load is suddenly applied or released)