What a delightful sentence! Let's break it apart.
The man who is set on securing reforms
You are correct that "is set on" means to be determined to do something, but it's important to realize he's determined to secure reforms. The word used in this way almost always has a political context, and securing political reform frequently requires considerable effort because it requires the support of a voting constituency — and people often do not like to change. So, the man is determined to obtain something that is difficult to get without the support of a group of people.
an ambitious windbag
This is a person who is a proficient and prolific speaker, but he is ambitious, meaning he will put his own goals before the needs of the people. Such a person will often act with greater enthusiasm due to the personal reward obtained by securing the reforms regardless the value of the reforms to the public.
desires the public good without possessing a ready tongue
This is a person who wants the very best for people but is a poor speaker (without possessing a ready tongue). This questions the value of a dedicated person who cannot achieve the desired goal.
So, we could express the idea of your example like this:
If you want to lead people well, choose a representative who speaks well despite his lack of ethics over one who has admirable ethics but does not speak well.
I'll leave the question of whether or not this represents valid wisdom to Philosophy.SE.