There will be much out there written on this subject. I think that in assessing the relationship between both characters, one has to notice a shift of weight. At the start of the play, Othello holds a great deal of power over Iago in terms of the latter wishing to receive the promotion from the former. Yet, when Cassio is favored, it is interesting to see how the relationship changes. Rather than become disempowered, Iago actually gains more power in terms of what he is able to do to others with his manipulation of them. It might even be argued that he gains more power with his demotion than anything else. I think that a comparison of both characters focus in a study of insecurity. Iago is insecure of Cassio's position and his apparent favoring by Othello. The decision to promote him over Iago triggers in much of this insecurity, causing him to embark on a manipulative quest. Othello's insecurities are much more subtle, not really as evident until things start to work in Iago's favor. He is insecure of his ethnicity and age, his status as a soldier who has gained "insider" status, and the fact that he cannot believe that someone like Desdemona is in love with someone like him. In the end, both Othello and Iago are haunted and fraught with insecurities, making them more susceptible to enduring pain from deliberate and unintentional acts of cruelty.
Othello is the protagonist of Othello and Iago is the antagonist. While Othello is a tragic hero who is a generally noble and admirable man despite having the tragic flaw of jealousy, Iago is an unmitigated villain.
Othello is passionate and impulsive while Iago is cool and calculating. In Shakespeare's time, part of this would have been accounted for by racial and national stereotypes.
Othello is a Moor, meaning of Arab descent and born into Islam as a religion, while Iago is a native of Venice. This makes Othello possessed of a dark skin and Iago paler, something mentioned many times in the play. Othello is older than Iago.
While Othello is a general, Iago is only an ensign. Both men are married to loyal and loving wives, but both ultimately mistreat or betray them, Othello by mistrusting and then murdering Desdemona and Iago by his evil nature.