I'm an oils guy, myself, but I do use acrylics - not for outlining. Oils dry slow, achieve an even, consistent flow, and are correctable (with a turps-damp brush) for a long time. Just be sure the undersurface is completely dry, and the oil paint is well-mixed and about the consistency of milk. Using a liner brush helps too.
I'm sure there are ways to get this same effect with acrylics, and someone will no doubt post that method here, but I find oils easier and quicker.