Is the Invisible Man or Ras the Exhorter more convincing as a speaker and leader and why?
We might argue that Ras the Exhorter is a better leader than the novel's protagonist (we can call him the Invisible Man, as you've done in your question). Ras is the better leader in large part because he wants to lead and wants to inspire direct action.
The protagonist takes a position with the Brotherhood as a spokesperson. The political group uses the Invisible Man to disseminate their doctrine and to organize the people of Harlem into a structure that might potentially be activated and called to action at some point. Yet, to the Invisible Man's disappointment, his work as an organizer is essentially discarded by a change of policy in the Brotherhood.
In effect, all the speeches, the teaching, and the organizing undertaken by the protagonist never get anywhere. His work does not point the people he speaks to in any specific or actionable direction.
We should also note that political leadership is not the Invisible Man's aim when he arrives in New York. He only wants a job and accepts his position in the Brotherhood because the organization (in the form of Brother Jack) flatters his ego at a time when the Invisible Man has lost his way in the city and lives on the charitable feelings of his landlady. The political views of the protagonist develop only after he takes a job with the Brotherhood and, even after this, his views continue to evolve.
As a leader, the Invisible Man would seem to lack conviction -- a trait that sets him apart from the highly politically invested and self-assured figure of Ras the Exhorter.
Despite this distinction, Ras may not be the better speaker. The Invisible Man utilizes a variety of persuasive techniques to energize his audiences. In almost all of his speeches, the protagonist articulates the oppressive, disdainful and exploitative perspectives of a dominant class. In doing this, the Invisible Man creates a situation that challenges the dignity of his black audiences (and in his first speech demonstrates a sympathetic and exonerating understanding of a racist oppressive class in the South).
Ironically using the "voice" and logic of the dominant class, the Invisible Man achieves a more nuanced and complex argument than Ras ever does. He also generates a lasting sense of resentment as opposed to a potentially short-lived anger.
An unintentional irony plays into this scenario. The protagonist is unsure of his own identity and it may be this uncertainty that allows him to articulate the views of the dominant group that he works to overthrow (through the political work of the Brotherhood). Early in the novel, that protagonist's conflicted relationship to the dominant group (and to the idea of the dominant group) is dramatized in the episode with Mr. Norton, the wealthy patron of the college.
Norton is both a source of hope and threat for the protagonist and this dynamic functions as a concise example of the protagonist's conflicted identity. Yet it is this very confusion that gives the narrator insight into the thinking of the dominant group. An explanation of part of the power of his speeches, this perspective also explains the Invisible Man's lack of ability to lead. How can he be a leader when he is beset by self-doubt?
"Left alone, I lay fretting over my identity. I suspected that I was really playing a game with myself and that they were taking part. A kind of combat. Actually they knew as well as I, and I for some reason preferred not to face it. [...] I imagined myself whirling about in my mind like an old man attempting to catch a small boy in some mischief, thinking, Who am I?"