$.02 more...
i ran my 10 x 50 Acra knee mill, knee driven, pc controlled and built by CNC Automation here in NH USA, for 11 years, with NO problems related to the knee driven part. in fact having the spindle free was the best part. position over a hole, and mdi hand tap holes small as #2-56 with the tap held in a drill chuck. or lift the spindle and watch the program run with cutter above part. also enables you to use a quick change tooling like ultron snapchange, or a power drawbar and TTS type tooling. no its not a bed mill, but with high positive insert cutters, moderate cut depths and fairly high feed & speed, you can move metal, including steel, at a pretty impressive rate. if i was doing a lot of tiny short move work, or engraving, id like the quill driven. otherwise, no. my acra has 15"+ of cnc Z
it has ballscrews, fat ones at that. so i dont have to deal with a cnc with old acme screws. but having said that, i worked with a company for years that only employed big clunky worn out machines. one of these they retro'ed with an old clunky cnc system. despite the large amounts of backlash and way slop, this machine never sat empty. they found ways to program so the looseness wouldnt crash the machine. of course it could not machine a 30" dia +/- .001 hole, but folks lined up to have that hole put in at +/- .015 tolerance.
and im lucky to have dc brush servo motors already mounted. but i think 50 ipm of stepper bridgeport would be great, when hand cranking is the alternative.
i did say my acra has 15"+ of Z. well no it doesnt. it is electronically brain dead. old like me heh. but im working on it now with the help of a forum member, in a year or less i hope to see it uccnc guided, and using Mach too, and a better machine than when i bought it. i did a lot of great stuff on her, hope to have her ready when i retire from working for the man which ive been doing for 6 yrs or so, the man has been kind enough, but working for me i like the boss a lot better.
then, after all this fun, theres the lathe in the corner.........