[stone of vision]

Both of them could be valuable leaders, but each of both in a different way.
Boromir has for him, his passion, his valiance, his audacity, that self- confidence that force admiration, and a stringent reseach of perfection which seems to obssess him.
As Gandalf he was a "masterman", a man who seeks to rule without compromises, his destiny and those who would follow him.
That makes him vulnerable to the eseay side of the power, and the bite of pride.

Faramir has inherited of wisdom and never wished to lead, though his bravoure is a equal as his brother.
His concern, his care for the well -being of the people who live and from where he comes, his likes for what seems right, will give the strength and enough motivation to fight until victory.
He is one of those, for who power comes to them whitout them seeking it. Since they have no choice, but to use it in productive ways.

Because I tend to be more sensitive to the second description which correspond to my convictions, I would choose Faramir for that reasons et not because he's the best.

By the way, none of them is doomed to rule, as the place is deserved for the King who shall return.