Japanese cotton has been processed in a way that causes it to wick much, much better than organic botton balls, which are great at absorbing liquid but *not* great at wicking, as anyone who has ever filled their RDA up and seen lots of wet cotton yet the stuff near their coil is dry. It's pretty great at it, and if they were the same price, I'd use it, but I already have a giant bag of swiffers organic and I build my coils around it and haven't replaced the Japanese when I ran out.
So yeah, you'll get better wicking which can improve flavor, but due to the way we build coils and the fact that it still doesn't wick great... if you are seeing a night & day difference, you're probably wicking your cotton balls wrong.
Rayon & Japanese are neck & neck in all the testing I've seen, though rayon holds up much better with really dark and sweet eliquids than cotton or japanese cotton. Stuff seems to take a much longer time to gunk up the wicks when I tested it, which isn't a joke if you're loving a dutch chocolate or mocha eliquid and feel like you're needing to rewick every 6 hours. It also seemes to "let go" of your previous flavors faster when you switch without rewicking. At the same time, this stuff is probably not what you want to inhale if it gets at all dried out or your coil is hot enough to be scorching it.
Side note: I always assumed Rayon was a man-made fiber of some sort, ala spandex or some weird spun petroleum. Researching showed it's actually a highly-processed & highly-chemically treated wood pulp dissolved in acids and re-extruded out as fibers. I think. Always possible there's different rayons.
Personally, I think the biggest bang-for-buck upgrade over organic balls is rayon over japanese cotton, especially if you like darker & sweeter stuff. You get almost all the vaping benefits as Japanese cotton, while holding up a lot longer without rewicking and keeping flavor. Not a small thing but still a bit wigged at inhaling it, but that could ignorance on my part.
