Yeah, the size ranges I've compared APC props in is 7" up to 12", though not all sizes in that range, and not all in what I would call "consistant" conditions. I've no clue about props in the 20" range, things could easily change quite a lot in different size ranges.

Flexing of the prop tips is a bad thing, not a feature. It causes noise and burns off energy in movement and noise instead of moving air. I agree that the smaller APC props do load the engine more, so, if you want to unload it, go to a slightly smaller prop.

It's also worthwhile to keep in mind that RPM does not equal power output. I've found that turning an APC prop at a lower RPM usually results in better all around performance than a screaming MAS.

There are, however, exceptions. I run MAS 9x4s on all my .25 and .28 combat engines. An APC 9x4 is too much load, an APC 9x3 isn't quite enough. But combat flying pushes the envelope in different ways than some other types of flying. And then there is the durability issue. Combat flying has some unique prop durability "issues" that other flying doesn't have to deal with

So you really should pick a prop based on your specifc uses. Buy one of each, fly em, and see. That's the ONLY way to know for sure.

My OS .32F engines on a Macs pipe or mousse can just LOVE the APC 10x5. more thrust than you know what to do with, and plenty of speed.

YMMV of course.