My Mag .25's with mousse can seem to like the APC 9x4 best, though it will bleed some speed in turns this way.

The Mag .28 is a much more powerful engine than the Mag .25, and it likes the MAS 9x4. I also run the KY mousse can on them.

Generally, APC props are more efficient, but load the engine more than the same size MAS. I think I read somewhere that the MAS props are actually pitched differently, so that a 9x4 MAS is roughly the same load on the engine as an APC 9x3. If I had a 9x3 APC, I'd try it to see what happens. But I don't, so I stick with the MAS 9x4 on most of my combat planes.

The lower pitch results in higher RPM, which actually makes the plane fly faster than with a higher pitched prop, and the 9x4 won't bleed speed in turns like the other props, and in combat turns are where it's at. Straight line speed isn't very important anyway.