The difference between steppers and servos isn't accuracy, per se. As long as a stepper motor is operating correctly and doesn't encounter an obstruction, it performs just as well as a servo. However, if the stepper system jams or misses a step for some reason, the controller will continue on as though nothing has happened. It never knows that the motor is no longer in the correct position. That aside, I've been using a stepper-based system for about a year and a half. It's never missed a step, and accuracy is limited by the mill itself, not the steppers.
Sorry, can't advise you on the various turnkey systems. I bought a CNC-ready taig from Nick Carter and picked up the various pieces from the respective vendors. I highly recommend the Gecko 540 4-axis controller.