Iago is the jealous one. He worries that Othello is encroaching on his territory. He is frustrated and angry, and basically nervous. Othello is paranoid, which is not really the same thing. Iago's jealousy is real, Othello's is imagined from his own arrogance.
"Jealous" is a sometimes misunderstood and misused word. For clarity, here is a simplified definition for this complex concept: Wary of being supplanted in affection or position; vigilant in guarding possessions; intolerant of disloyalty or infidelity (American Heritage Dictionary). Iago is wary of losing his position; of Othello advancing other soldiers over him. Iago is vigilant in guarding his possession in the position in his career that he has attained. Othello becomes vigilant in guarding his "possession" in Desdemona after he hears from Iago of reason to suspect her. Iago and Othello are both intolerant of disloyalty or infidelity. Iago is intolerant of what he considers Othello's disloyalty to him. Othello is intolerant of Desdemona's imagined infidelity. It seems the scale tips toward Iago being more jealous than Othello, even though both men commit terrible acts as a result of jealously.
I think that Iago is first and foremost a misanthropist: there is no-one he cares for more than himself. Iago's feelings and emotions seem so different from any other character in the play that he is hard to compare with anyone. Othello is consumed with the idea that his wife shows affection to anyone but him. His jealousy is deep and all pervasive.
I would agree that Othello is perhaps the more jealous individual, but for somewhat different reasons. I think above all Iago is defined of his hatred of Othello rather than his jealousy of him. This is what leads him to commit himself on his course of destroying Othello. Othello, when the spark of jealousy is first kindled, finds himself completely dominated by the emotion and is unable to control it in any way. It is his jealousy that forces him to react in such a disproportianate manner.
I think it's Othello. The reason I say this is because he becomes jealous with less reason than Iago. For Iago, jealousy is at least somewhat logical. He is subordinated to Othello and he thinks he should not be. That's a "good" reason to be jealous. By contrast, Othello has no real reason to be jealous over Desdemona. Since Iago's jealousy is more logical, I would say he is the less jealous person overall.