Slow Stick
You've asked a good simple question, which unfortunately has no simple answer.
I don't know what gearbox or prop GWS supplies with the EPS400C drive system in the Slow Stick. Therefore, I really can't answer your question about which is more "powerful". The EPS300C powered Slow Stick flies fine. And appears to be the most common version of the plane.
Check out GWS's website for all the information on how their different motors and gearboxes will perform under different voltages. But keep in mind these are only close numbers. The real world will be slightly different.
Which battery will last the longest again is specific to the motor, prop, and gearing of the plane.
As a general rule: the higher the MAH rating the longer the battery will last for any given amp load put on it. The amp load you put on a battery pack will change due to your flying style and the day to day flying conditions (temp., wind, etc.)
But it must be stressed that not all batteries are created equal. Some batteries are capable of suppling more current than others and as a result not all batteries are acceptable for all applications. The airplane must also be able to lift the battery pack. It is possible to get a motor which will run for an hour, but the airplane will never fly due to the fact it weighs too much.
When starting out. I suggest using the suggested battery system listed by GWS or by others here on RCU which have already been found to work.