IMO if Mcgregor can knock tf out of someone in the 1st round and doesn't have that cardio then you'll know a Good Chin is gonna make you feel like you can squash a potato but then again depends on your fight set, if you wanna go aggressive, have a good chin and knock tf out of people by swinging for the fences or if you wanna play with your food then cardio's your go. (forgot to mention you don't have a choice, you born with what you got)
Also imo, I'd rather have a good chin. mainly because I love to get more violent than to last 5 rounds in the tank.