Well, I'm not sure there is an easy answer to that.
When the car is cold, oil pressure will be up around 100psi, so naturally the system will fill the accumulator with a some volume of oil that reflects the position of the internal piston and the compression of the air chamber.
With the EPC valve, this volume can potentially be maintained as long as the system pressure does not drop below a certain point. However, in practice the idle pressure is low enought that the valve will open, and now you are limited to the capablities of the system to refill it. With warm/hot oil system pressure will only reach somewhere around 60psi at 5000rpm and around 80psi at 7000rpm. (if I remember correctly, it's been a while since I've driven my 'tour)
So... what we see here is that assuming you don't drive the car hard all the time, normal driving is only going to fill to a volume corresponding with < 60psi, and thusly when taking a right hand sweeper at a high enough rpm to cause pump cavitation (~4000+ rpm) the system's oil pressure and the pressure in reserve are going to be essentially the same.
My point: In theory, on a contour, the volume of oil in reserve will be nearly equal with either valve, at the time of typical system pressure loss. Personally, I see no benefit in using an EPC valve on a contour, on a performance car with a race pump pushing higher psi's at lower rpms yes. There is the potential to have more oil in reserve, but only enough to compress the air chamber an additional few psi during normal operation, which will be a minimal amount.
If using the EPC I'd say yes, you the 20-25psi would be best for use on a contour.
*personal opinion warning - might be wrong* I believe that the EPC may actually be more dangerous to use than the standard electric. heres why:
Say you are using an EPC valve that opens at 25psi, and you are taking a right hand sweeper at 4500prm, so system pressure is about 55psi. Typically, the rule of thumb is that you should have 10psi of pressure for every 1000rpm in order to maintain a sufficient oil barrier between moving parts (ie rods and rod bearings) thus far our example is above optimum, now oil pickup looses submerission, pressure drops to basically Zero (normal contour problem) at this point the EPC sensor triggers the opening of the valve, valve opens, oil repressurizes the hose, then the engine.
Comare to same situation, but standard valve, pressure drops, oil flow referses direction in hose, engine has oil.
Although not much, and quite probably insignificant, the EPC valve has the potential to introduce a delay in response, since the sensor must detect the pressure drop, open the valve probably not even 1/5 of a second, but then the oil reserve must also account for the fact that during that 1/5 of a second oil will have been draining out of the hose thanks to gravity.
As I stated before, this is my own personal opinion, and it may be inaccurate, not to mention the time needed to open the valve may not make any noticable difference, these are just my thoughts on the matter.
Accumulator Operation Diagram
