But as soon as you switch to end mills (per the the OP's requirement),
you lose a lot of the horizontal's rigidity. I.e., the cutter is no
longer supported at both ends. Bob

I think a horizonal mill is still much more rigid than a vertical
mill for a number of reasons. First, even my small clausing horizontal
mill have a #30 taper which is much heavier than an R8 or MT2
vertical spindle. Second, the spindle is in the column which is
directly attached to the knee. A lot more stuff is between the
table and the spindle in a vertical mill.

Using an endmill in the horizontal I can plung with the Y axis,
which is probably more rigid than plunging with the quill on most
vertical machines and definately more rigid than the quill on a mill
drill. I know because I got the big Jet mill drill and its no
comparison to my rockwell horizontal. Another big plus is that the
rockwell has almost 7 inchs of Y travel which is more than any
vertical mill I have ever used.

Vertical mills are very useful, Horizontal mills just rock.

chuck