I always saw it as constructing versus running models. Historians dispassionately collect and interpret data. In doing so they can come to surprising conclusions, e.g. around how violence [1] or hegemony [2] propagate. Counterfactualists run these models to simulate decisions, both past and present.

[1] http://www.amazon.com/The-Better-Angels-Our-Nature/dp/149151...

[2 http://www.amazon.com/Tragedy-Great-Power-Politics-Updated/d...