6:1 or 8:1 with a stepper will have good holding power, but it may not have the resolution you are after.
With a 1:1 ratio, each step of the motor is 1.8 degrees. With a 6:1 ratio, each step will be 0.3 degreess. With an 8:1 ratio, each step will be 0.225 degrees. With half steps, those degree numbers half. With microstepping further the same occurs.
If you assume full steps, and a 3" diameter work piece, each step would be:
- 0.047 inches of circumference travel per step if 1:1
- 0.007 inches of circumference travel per step if 7:1
- 0.005 inches of circumference travel per step if 8:1
Again, those number shrink if you half/microstep, but you will also lose torque. I decided to go with a harmonic drive coupled with a pulley ratio to get 66:1 which should give me awesome resolution regardless of workpiece size. For reference, i believe HAAS' 4th axis is 88:1. Also keep in mind that with a stepper you probably wont be able to spin it anywhere near fast enough to do any spindle work like Simpson is doing with his Servo setup. Lastly, 8:1 may be difficult to accomplish within a specific given space. You will probably need to have a multiple pulley setup which can potentially introduce more backlash.
A servo will allow you to get much higher resolution with much lower of a pulley ratio, and will also allow you to spin it up fairly fast. The downside is servo's dont have much holding power, which is why Simpson introduced a brake when doing 4th axis work vs lathe work.