I only have single instances of the three, but for me the FA is the softest, then the SF, then the SF in my Falcon.
They're all completely different beasts. The Falcon pens (aka Elabo) have completely different geometry and get their behavior mostly from the geometry, but I don't find mine to be "soft". I would describe mine as being rather stiff, but with a geometry that is intended to encourage more spread between the tines when you push on it. The FA nib, regardless of whether "FA" stands for "Falcon", are a completely different geometry from the Falcon. Then the SF nibs, they are a traditional nib geometry, they're slightly more flexible than their non-S counterparts: they require less pressure to get the same pressure and, I assume, are less likely to fail when you do that repeatedly.
Then there's the size-30 nibs used in the Custom Urushi and Aya. Traditional geometry, bigger than the KoP and 149. I would put it's softness between my 743-FA and SF which IMO is perfection.