A lot of that is difficult to answer. Subjective experience is always best with these sort of questions.
The best stepper/board KIT that I know of is a xylotex. There are likely many other, more expensive kits available that could be thought of as better.
As far as I know of, CNCBridges has not yet delivered.
I've milled with my Manual X2 into the +/-.001 range, but it's a difficult prospect. My machine requires constant attention and re-calibration for those results. Mine is from Harbor Freight. It's possible that some with X2 machines from Syil or Grizzly are experiencing better and more constant results.
Generally, for those sort of results, you'll want a few machines to do the work. A 50 year old surface grinder would be quite beneficial toward getting extremely flat surfaces without taxing your mill too much.
My point is that the X2, if used commercially, on steel, should be used as sparingly as possible to promote best reliability.