I would also like to point out that your stepper analogy is not as accurate as you think. Microstepping is not accurate, it is somewhere between the commanded microstep and the one next either way so at the very best all you can say you are +- 1 microstep and that is before you take any of the parts of the machine into equation.
If your servo system has 500 line encoders then the theoretical resolution would work out to the same as a stepper system with 10 microsteps, which would be more accurate would depend on the tuning you can achieve in your servo system. If you have higher count encoders 2000+ line count then, assuming your servo drives are capable, you will have a much better chance of having much more accuracy than you could hope for with stepper.
However as I mentioned before a Bridgeport is not a rigid machine and personally I would be happy at +-0.0005" accuracy.
Hood