I'd call it a dead heat as far as full backs go, with each being elite and well adapted to the game style of their era. I think SOS was the better all round player because he could play forward too.
SOS was far more versatile. Could take Dunstall at FB, Carey at CHB, Hird in the midfield and kick 10 up forward. He was a better fullback IMO, one on one, head to head, I'd rather no other I've seen, but I understand that's an arguable proposition for some. As a footballer though, it's not even close.