Spindle Motor - Stepper v DC Motor

Hello all;

Need some pros and cons regarding switching from my Sherline DC
Motor to a Stepper Drive.

Obvious advantages are easy speed & direction control which makes
tapping & auto tool changes possible, what other advantages /
disadvantages are there ??

If I was to replace the 1/2 hp DC Motor with a stepper, what nema
size, holding torque & drivers would I require to be equivilent in
terms of torque ??

Many Thanx

PeterC