A vfd will require some wiring on your mill but will give you infinitely variable speed control and some other options
I know I'm late to the party here, but I question the above. Unless AC motors are designed for VFD use I believe running them with a VFD can be detrimental to the life of the motor when run out of spec.
However, if the VFD control was set up to send the spec power to the motor (not vary the frequency to vary the speed) I can't see how that would be any different than service 3 phase, as the motor would see it. If the object was to vary the RPM of the motor - then my same thought above is there - unless a motor is spec'd for VFD control I believe it will shorten the lifespan of the motor.
In the case of a large machine like a Bridgeport mill - the motor is something I would want to take a lot of care of and not do anything that would expedite its' demise.
I know on smaller machines - gear drive lathes for example - VFD conversions (controls AND 3-phase VFD-compatible motor) are popular, but are still expensive conversions.
Another, possibly off the wall, thought -
If there is a possible future application in the shop for VFD control to control the speed of a motor on a machine - then the investment of the VFD unit might be a better long-term investment than a RPC, or SPC. Though, if what ever option is chosen is to be married to the mill and never leave then it might make the decision a bit harder.
Food for thought anyway.