Robhi,
Can't speak for the others, but I am confused about exactly what your configuration is, especially what 'spindle' you refer to from one post to the next.
My impression is that you have one high speed spindle which will be the 'pen' and do the actual engraving and the second spindle is holding the workpiece and does not need to do anything other than slowly rotate the workpiece in the manner of an 'A' axis for a mill.
If the above description fits your machine, then the remaining question is how fast you need to turn the 'A' axis. If you need to go fast for wood TURNING, you need a servo. If you only need to go slow for wood ENGRAVING then a stepper would be adequate.