First of all, try cotton. Just try it out. Here's why I say that - I vaped for over 1.5 years and never tried anything other than cotton. I FINALLY tried Rayon that a kind member sent to me, and, well, I'll never use cotton again - BUT - as you see from the mixed replies, it is more than worth experimenting to see what YOU prefer.
I prefer rayon because
-no off taste. People who get an off taste don't wick it correctly. That's not an insult, just the truth...(same is true of cotton, actually, so... this isn't a pro over cotton...if you taste cotton...you wicked it wrong, sorry, try again)
-greatly extended wick life over cotton, leaps and bounds.
-WAY better flavor, YMMV, for me it was enough of a difference to promptly change my screen name to "Rayonati" Sloth...
-I also think it is easier to wick it correctly than cotton, BUT, both wicking materials have a learning curve. And IMO most people (say, in youtube videos) use too much cotton, so there's that...
-All wicking material is inexpensive, but rayon is extremely cheap. That's NOT a reason for me to prefer rayon when KGD costs very little as well. But, it's a pro, since I prefer rayon anyway.
-NOTHING is conclusive with safety. But
in my opinion based on what is known rayon is clearly a safer wicking material.
There's actually not a single reason that I'd consider cotton better than rayon. Rayon is superior in every single way, TO ME. But you'll see people say the exact opposite, my friend. So, try cotton. Specifically Koh Gen Do is what *I* used and found it superior to all that "made for vaping" crap. But, a lot of people prefer, say, Native Wicks...
Be it wire, wick, juice, high PG or high VG, ohms, or
any aspect of vaping - YMMV, so it's worth experimenting
ETA I shouldn't have said that all wicking material is inexpensive...that may not be true when you get into "specialty" wicking materials like Native, unsure.
One more ETA bro, if you're interested, PM me. I'd be happy to send you some KGD pads to try out in an envelope