I don't know how big the mill is that you are talking about. Best bet for me
has been go with what is known to work. 800 to 1200 oz-in steppers for a
full size BP is comfortable.
I am running 200 oz-in servos, 4:1 reduction on a full size series 1 BP
and it will cut anything I give it. This gives me 800 oz-in at the ballscrew.
Servos also have a bit of reserve torque that you can tap for just a small
bit of time if needed.
I say go a bit larger with the steppers because if you "lose" a step
you will never know it, plus there is no reserve torque with steppers.
Of course you can gear the stepper to increase the torque but you may
be dissatisfied with the speed.
Scott