Get a Model F and take it apart (the big 122s are much easier). You will see that it is, indeed, the Mac Daddy and that the M was a cheapened downgrade.
The only major downside I see is that the plastic case is more brittle, and this is an even bigger problem on the F AT, because of the way the case snaps together, especially at the front (spacebar) side.
The M probably used a softer, cheaper, plastic case, but its flexibility makes it much more forgiving.