AC servo's have just about replaced DC for most commercial systems, as for accuracy at low rpm etc, alot depends on how they are commutated and controlled and how many poles etc, e.g. sinusoidal or stepped sinusoidal have equivalent performance or better (lower inertia) than DC brushed.
if you have (BLDC), Brushless DC, they can display some cogging at low speed, but even these will can be very accurate if any kind of reduction is used, then cogging is not usually apparent.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.