question about servo amplifiers

I have some brush servo amplifiers.

A.M.C. 25A8K and Copley 422

Both of these have switches for four modes:

current mode
voltage mode
IR compensation mode
tachometer mode

Which is the best mode for use with a motor with encoder on a cnc
mill running EMC with a vital systems PCI card that outputs +/-10V
control signals and reads encoder information from the motors?

I guess what I am asking is with a setup like mine, how would these
drives in say current mode with my non-tachometer motors compare
with the same drive in tachometer mode with a motor that has a
tach? If EMC and the vital systems card is closing the loop around
the motor and driver anyway, what does the tach feedback to the
amplifier really buy you? Is it that it allows a faster response
time? If so, what type of difference are we talking about?

I am trying to decide if it is worth my time to develop a digital
tachometer that generates a voltage based on encoder readings.

Thanks for any input.

Rick