As Aus Aaz said above, Overloads are really mainly for PvMing such as Ne, Corp, and GodWars. However, some powerslayers also use Overloads to decrease the amount of time it takes to do their tasks.
Extremes are better for general training and slayer though imo as use of the Prayer Berserker just keeps your stats boosted longer than the 5 Minutes an overload would last for.