Even though xanxus is more capable of handeling the vongola and the mafia, I still think tsuna will make a better leader..
for the following reasons
he would make a more diplomatic leader as the 10th for the vongola, he would balance the power and not go forge a bloodbath just for the sake of it.
he does care about the vongola despite running away from it and not wanting to be a apart of it so much to the point that he rejects it for the sake of not wanting to continue the ruthless bloodshed of the past vongola.
tsuna cares for his family, and his guardians so much that he would give his life for them- only a true leader would be willing to this.
he is vongolas true heir and has the blood of the vongola descendents running through. and it’s obvious that the 9th chose other than this obvious reason, I feel that seeing xanxus behavior and and his psychotic lust for nothing other than power made him seek tsuna even more believing that he would not continue the bloody and violent history of the vongola.
tsuna may be a pussy at times but he does have the real balls when it comes down and he will defend his right as well as the lives of his friends and guardians to the very end. he does have the traits needed to be a true leader. he has matured over the course of the series
plus I just feel from reading the story that tsuna’s character is far more interesting than xanxus, plus he has a personality and he’s not a complete heartless bastard who only seeks power to improve his false ego and low self worth