It's not that simple.
It depends on how fast you'd be spinning the stepper, and the servo.
A 1200 oz stepper probably has maybe 400oz of torque at 500rpm, and about 150oz at 1000rpm.
A 400w DMM servo has 170oz of torque at 3000 rpm. If you gear it 3:1, that's over 500oz of torque at 1000rpm, or about 3x that of the stepper. And the Servo has a peak torque about 3x higher, so for short bursts, like during acceleration, it can be nearly 10x more powerful than the stepper.
Depending on how much weight you are moving, and how fast, the 750wmay be a better choice.