The answer to this question really all depends on what factors you are basing your decision.
In order to determine how you want to evaluate each man, you might ask yourself some questions about them. For example, is one more moral than the other? Is one a better politician? Is one more loyal than the other? How will each man be remembered by others?
Both men have their good points and their bad points. Caesar was a skilled and respected leader and military man, but he became power hungry toward the end of his life. One might argue he was overly ambitious and that is what the Senator assassins were responding to. Brutus was well respected and moral, but he also helped to murder one of his closest friends. Perhaps he should have first tried to have a good honest conversation with him?
Because Caesar dies early on in the play, you might also take into account that you have more first-hand knowledge of the type of man Brutus is than you do of Caesar. Much of what we know about Caesar in the play is told by others, some of who have good reasons for killing him. Can those accounts be trusted?
Ultimately, whichever many you decide is the "better" one, you should be able to provide clear, first-hand evidence to support your claim.