Milling what? Foam- no problem, wood-raise it up, aluminum -advanced…If this is the main use ditch the printing unless you will be printing short parts.
I use mine for both. I know the largest piece I will ever print on it is the roller F so mine has a 5" Z. The downside is when I go to mill things I always put the pieces to be milled up on blocks to get them as close to the gantry as absolutely possible. You can use the adjustable leg feature to counteract this to get a longer Z, but trying to get a 12"z is not really feasible. You need to choose which one will be used more.
Trade offs. longer z taller machine = less rigid, slower, less precision.