Suarez admittedly is a diver and a little snake. That aside, he is a more complete footballer with 'world class' dribbling, heading, finishing, and creating skills. His ceiling is higher than I believe Aguero's has ever been. Messi, Iniesta, and CR are 1st tier, while Suarez is 2nd tier with Ibra. Aguero at his prime would be 3rd tier for sure.
Aguero is more physical, has a more straight-forward 'meat and potatoes' style of dribbling and is pretty easy to disposes (think CR). He is a menacing forward who has an eye for goal and is intelligent at using the field of play to his advantage (he cuts off angles and closes down spaces better than most). This is the type of forward you want on your team, but there are things that Suarez can do with the ball that Aguero just can't, and that is what separates the two.