About Dayne v Selmy
I think it's simple
Arthur got a more "Legend" imagery, not only he was absolute peak as swordman, but got the "magic" (as valyrian steel being special) sword support that... not a case he got Arthur (with his invincible sword excalibur) as name, GRRM just love wink at our historical/myth figures building his universe
Selmy was a more grounded char... he was incredibly good as you can get, but "real"
I ever associated him more with William Marshal
https://en.wikipedia.org/wiki/William_Marshal,_1st_Earl_of_Pembroke
Selmy was a 10, Arthur was a 10+1