Box Collider Vs Sphere Collider/ overlay

I'm creating an RTS and I need to know what is easier for the computer to compute as a trigger - a box collier or a sphere collider? or like a sphere overlay whatever that is?

Sphere.

http://forum.unity3d.com/threads/34254-Capsule-vs-Box-Colliders

I think there is no (measurable) difference in calculating those simple primitive colliders. You can also combine them to create compound colliders.

Physics.OverlapSphere is a function which checks against the bounding volumes of the colliders touching or inside the sphere.