knee mill router

I would like to know the advantages of on CNC mill design over the
other. The first machine is a Knee Mill (e.g. drill press converted
to a CNC machine) and the other is where the gantry is stationary and
does not move (see sample picture at the bottom of
http://www.campbelldesigns.com). If I want to mill precision metal
projects where precision is very important, how much accuracy will a
Knee Mill provide over the stationary gantry design? Also, what is
the advantages/disadvantages of one over the other?

Thanks,