4 Velodyne HGS-15's or 2 Velodyne HGS-18's?

Given this choice, which route, and why, would you go?
Building a theater and will either go with a pair of HGS-18s on either side of the center, and on the insides of the outter speakers, OR 4 HGS-15s. They would be placed in the same position, but 2 HGS-15s would be stacked on each other!

Thoughts?

For what it is worth, a single 15 would probably be sufficient for the room, but isn't 75% of what we buy overkill anyway?

Thanks for the insight.
Dan