Hi Carbonius !
Your questions are very detailed and specific and I think you know that
you are the only one who can really be trusted to answer them !
What I mean is that small differences in tone, volume , feel, etc. are all very subjective and personal.
As you state above, you could get an X20 and be completely confident about selling it on quickly if it is not to your liking. I guess the same is true of the Rainsong.
Dropping a couple of hundred dollars for an really extensive home trial seems OK to me ,( as long as the kids have shoes).
I arrived at the Martin I'm now happy with, by being able to have several at home at the same time for unhurried a/b/ testing.
Got them all used , mostly eBay, Flipped the ones that I eliminated.
Yeah,,, lost a bit of dough ,, nothing heartbreaking, but properly cured my GAS (for dreads anyway
)
Sorry, I'm not sure where my rambling is going here,
I think I'm saying get both if you could (temporarily) afford to.
Try 'em out for several months/different strings etc. THEN you can be sure which one's the keeper !
I have not played either of the guitars in question but recently got a used X7 and it is a beautiful instrument.
(Warning: I did end up keeping 2 Martin dreads!
)