Definitely I would go with Cassius! This is a rather cynical view of mine but let´s be honest. The reason why Cassius tries to persuade Brutus to join them is only because Brutus is a man who is famous for his honour and respectability by the people. Therefore he is not famed for his astuteness or canny qualities. That is definitely Cassius´ department, as Caesar himself recognises. Cassius has the cunning to size up different characters and read the way things are going, which Brutus, being honour bound, definitely does not. Unfortunately, as the play progresses, we see that Cassius deferring to Brutus leads to both of their downfalls.
Depends on how you feel, really. Let's ignore the decision regarding Cicero and focus on the disagreement on whether or not to kill Antony. Cassius recognizes for strategical purposes that killing Antony is probably the best thing to do. Cassius is afraid, rightly so, that Antony may be angry enough about Caesar's death to seek some retribution against the conspirators. With that in mind, if you believe in completely squashing a problem way before it starts, you need to kill Antony.
However, Brutus is concerned about the perception of the Roman people regarding their actions. He's concerned that if they kill too many people, their "course will seem too bloody." Brutus believes that by just killing Caesar, they can more effectively prove that what they did was for the best of Rome. If they kill Antony, they may look like knife happy murderers. So, if you believe in fixing the major problem and trying to fix the smaller problems, you'd leave Antony alive.
In hindsight, we already know they should have killed Antony. What would I have done? I probably would have agreed with Brutus at the time. Of course this means I would have tasted my own sword in act five also, but someone as high ranking and influential as Brutus could be trusted when he talked about swaying Antony to his cause.