Well, I'm by no means a SW expert, however in all my tanks (freshwater) I try to keep "Hardware" at a minimum (just hate the look of equipment in the tank) So, if it were me, and I could get 2 pieces that work as well as 3, then I'd go for the lesser amount.
However as Jim said, some corals may be more demanding of flow, to the point where you may have to add another unit down the road.
But again, I've read this in the stickies (I believe) enough flow is never enough, so go nuts, I'm running approx 800 gph in my tank and it's only 20g, I'm shooting for 1000ish we'll see.