Hey Tom, hey John, hey all,
I think the biggest difference is between using a hand trim router and a spindle at all. A hand trim router is made for short, hand-held usage, looses its warranty as soon you put it into a CNC machine, and because of the type of motor, cannot deliver high torque and high speed at the same time (what you definetely want when milling wood), will burn your workplace down (video) if you overload it, and has a runout beyond what you want to know, while a spindle is an induction motor often with double front bearing even on cheap ones, and because it is always driven by a VFD, can deliver high torque over a wide speed range, and a spindle is never in danger to begin to burn because the VFD will sense overload and prevent that. Plus you have no carbon-brush commutators on a spindle.
However there are still differences between no-name spindles and brand spindles: