tdracer,
2 spool vs. 3 spool and N1 vs. EPR are the big things.
Funny you bring up EPR verses N1 in thrust control, a debatable item amongst the engine producers.
Both Pratt and Rolls use EPR for control while GE maintains N1 (fan speed) gives the most control. I assume that on Pratt and Rolls engines, the pilot sets the EPR or core speed with the throttle that results in a particular fan speed.
But, GE says thrust is more accurately controlled by setting fan speed because all of the airflow is pumped by the fan rotor. The core rotor only handles a portion of the airflow. Since fan and core speeds are coupled, thrust can still be accurately modulated by scheduling core speed as a function of PLA (power lever angle). From data that can be obtained from energy balance between fan and core speed, a speed curve can be generated depicting corrected values for both fan and core.
Since speed is affected by various parameters within the engine design and external factors, GE concluded that core speed is not an optimum manipulated variable to set fan speed. Fan speed is controlled as a function of T2 (inlet temperature) which results in corrected fan speed control.