DX11 seems to use less RAM and more GPUmen, same as OpenGl does for me. But DX11 isn't stable for me when doing ALT+TAB from true fullscreen. Haven't tried the -popwindow combination yet. EDIT: I did try the -popwindow and it works with DX11, but It's the worst reformer of the 3, I get bad fps and "physics stuttering" when using it. So back to OpenGL which seems to work great so far, perhaps it helps that I have 3GB of GPUmem (GTX 780) since OpenGL seems to demand/allocate more of it than DX9.