#1 Do you like to be a late game carry? If yes, Yasuo
#2 Do you like to be more of an assassin or a tank? Assassin = Yasuo, Tank = Darius
#3 Do you like to be able to sustain better or do more damage at one time? Sustaining = Darius, More damage = Yasuo
#4 Do you like to be better early or late game? Early game = Darius, Late game = Yasuo (Hyper carry late game after a few core items.)
#5 Are you good with skill shots? Yes = Yasuo, No = Darius.
There are many more factors, but this is the main template I go buy when I look for new characters.