I use a horizontal warping mill and love it. Very easy to use and I would not trade it for anything.
If "bending over" to use it was a problem, one could easily elevate it by putting in on a table or putting something under it to raise it to a comfortable level. As a matter of fact, I would argue that it is easier on my back than a vertical mill because I am not bending up and down as I am winding warp.
The only valid argument I can see for going with a vertical mill is that it would probably take less space, but my horizontal mill folds for storage and has never seemed overly obtrusive or hard to work around as far as space goes.
I would also think that designing and building a horizontal mill would be a little easier project than a horizontal mill.