I haev a mag 12 and 9.5 both running in sump, my reactor has a mag7 running external. I personally would put them in sump, But if I could do it all over again I would run external pumps with greater abilities
. The Mag 12 has been very reliable, 5 years without ever touching it. If you do run a mag external you will definatley need a vibration pad of some sort.