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?
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.