The rockwell head has only 2.5 inchs of travel so the M head wins
in that area. The Rockwell has R8 so it wins in that area.

The only thing that would make me convert is power feed. If you
have a knee, why isn't 4 inchs of quill travel enough?

chuck

The Bridgeport M head came with your choice of three smaller collet
tapers (B&S 37, MT2 or Bridgeport's own C taper). it has 6 speeds on
a 1/2 HP motor, and no back gear. Also, no power feed. It is a fairly
light
milling head compared to the later J head, which has all those features
(power feed and back gear). The M head has about 4" of quill travel,
compared to the J head's 5". That extra inch makes a BIG difference
when switching between a 1/2" jobber's length drill in a Jacobs chuck and
a 1/4" end mill in a collet.

Jon