I think that Iago is more successful at influencing Othello. His methods are premeditated. He plans each step psychologically and is accurate in his delivery. There is a subliminal aspect about aspects to his work, with which he is able to convince Othello of Desdemona’s wrongdoing. He never admits his thoughts to Othello because he would be able to find an excuse or deny such direct implication. Instead, Iago makes Orthello think the worst, he is not able to deny his own heart.
Desdemona is much less successful in her attempt to influence Othello. She is straightforward and naïve. She takes it upon herself to get Cassio his job back. Believing that Othello will not resist her plea, she puts little strategy into her effort. Also, by not doing so she is forced to see him in a very childlike manner. This does not impress Othello, but Desdemona does not notice his displeasure or the fact that she is not having the right effect on him.
Iago is more successful because the plans each step. He makes Othello succumb to his own thoughts about Desdemona. It makes him so angry that he can no longer think rationally about the situation.
Desdemona is less successful because of her tactless approach. She is not aware of what Iago has said, and this plays into his hands when she is hassling Othello about Cassio. She only angers him more, which reinforces his suspicions about her.