Both are actually pretty smart, but Cartman has the advantage of being able to fool everyone so that they'll step into his trap at the end. Also, Cartman has shown political talent by giving various speeches and by making others believe and follow him, for example when he founded a Christian rock band. He is also very convinced of himself and is a good at planning things carefully.
Mewtwo is ice cold emotionally and wouldn't let anyone else talk him out of pursuing the goal. Mewtwo can read thoughts and has the ability of telekinesis and can also foretell the future with a 100% accuracy guarantee. Mewtwo could easily ward off all direct physical attacks if he were assaulted at a public speech for example by using powerful psycho superpowers.
Steve Rogers/Captain America Impossible to corrupt check Extremely Honorable in all his dealings check Cares about every single person on this planet and not just those with money. Check Loves the Ideals of this country not this countries politics, Check Utter badass, check