When it comes to flow in saltwater tanks, my impression is; generally the more (random) flow the better. In the aquatic hobby in general, the more redundant equipement, the better. So I would personally go with the closed loop + additional pump option. If the pump in the sump should happen to die prematurally on you, at least you still have the mag 18 going... or if the mag dies, you still have the sump pump.
Keep in mind, I'm still a newbie at this whole saltwater thing... so take my suggestion like a grain of salt (pun intended
).
-Richer