[Frank Lee]

The spoilers above the window are intended to reduce lift at high speed; IIRC they first showed up on a performance variant? If by chance they reduce drag it would only be because of the reduced lift and lift = drag.

The spoilers below the window are intended to reduce drag and lift; the reason these have a better shot at reducing drag being they promote attached flow across the back window.

The main difference between the two is in the size of the trailing wake. The below window spoiler should be leaving a much smaller trailing wake area at the rear of the car than the above window version.

I think running both is counter-productive; logic dictates that you'd want the above described effect of flow across the back window when running a low spoiler which a top spoiler would ruin. But a good test would be the final judge of that.

I'd run a low one because it should have less drag. I'd imagine the reasons for running a top one would be if lift reduction rather than drag reduction is the top priority, or perhaps for equal lift reduction the top one could be much smaller, or ?