I have both but the IP-A is preferable to me. Reasons are encoders are fed back to Mach so DROs are updated from encoder position. I switch on, home machine then as long as I keep the logic side of the servo drives powered the DROs in Mach always reflect the axis position.
For example I can disable the servo drives and manually move the axis and the DROs in Mach show the movement.
I can hit a limit switch and the DROs still show the correct position even if the axis moves a bit with inertia.
I can hit the E-Stop and again Mach knows where it is.
With the IP-S or any other controller that does not feed encoders back to Mach then you would need to rehome each time after doing one of the above.
Also you should be aware if disabling a servo or for that matter a stepper you need to re-home when you re-enable it as it has likely moved slightly due to forces such as gravity or belt tension, with the IP-A there is no need for that as it knows exactly how much it moved as the encoders are being monitored.
Hood