If I were Brutus I would want the better good of Rome, but I would also want to keep a good friend of mine. From the play, I can't really see this "GREAT" friendship neither of them have confronted themselves up until now, I don't really know HOW strong this friendship is and that's why I' d probably choose for the better good of Rome, and try pushing Caesar away. I wouldn't even know know what would be going on inside Cassius' head, I'd just follow Cassius, as he would bribe me and I could end up killing Caesar, not achieving ANY power and letting CASSIUS be king!