Servos don't generally come in NEMA sizes, they come in frame sizes. They are essentially the same thing, just slightly different standards with slightly different standard sizes. IIRC NEMA is imperial based, with frame sizing being metric based (it's been a while since I've actually looked at the specs!)

Closed loop servos do have their uses, for example where the lower speed is more beneficial than using a servo that would need gearing to achieve the same speed/torque than a direct coupled stepper motor.
However with cheap servos now available, the cost benefit of using closed loop steppers doesn't really exist anymore.

Each option has pros and cons, it's a case of deciding what is more important to you.