On the night before the battle at Philippi, over what do Brutus and Cassius argue in the play Julius Caesar?

Brutus and Cassius engage in a series of arguments on the night before the battle of Philippi.  When he arrives at Brutus' camp, Cassius begins the exchange immediately by saying,

"Most noble brother, you have done me wrong" (IV,ii,37)".

Cassius is angry because Lucius Pella, a friend of his, is being punished for taking bribes.  Cassius has written letters defending his friend, but Brutus has ignored them.  Brutus, in turn, chastises Cassius for defending Lucius Pella, and questions Cassius' own reputation as it relates to the matter of taking bribes.  The two men almost come to blows with their swords, but Brutus defuses the situation by reminding Cassius that they killed Caesar in the name of justice and not for their own personal gain.

Brutus then expresses his anger at Cassius, because he has requested money from Cassius to help pay his troops and been refused.  Cassius denies the charge with such vehemence that Brutus is appeased, and the two shake hands, reaffirming their friendship.  The two men then turn their attention to the coming battle, where, again, they disagree.  Cassius thinks they should wait for the enemy to come to them, explaining,

"'Tis better that the enemy seek us.  So shall he waste his means, weary his soldiers, doing himself offense, whilst we, lying still, are full of rest, defense, and nimbleness" (IV,iii, 198-201).

Brutus, on the other hand, thinks they should advance and strike the enemy first, arguing,

"There is a tide in the affairs of men which, taken at the flood, leads on to fortune...and we must take the current when it serves or lose our ventures" (IV,iii, 217-218, 222-223).

It is significant that Cassius accedes to Brutus' plans, following a pattern of submission to the younger man's decisions, despite the fact that Brutus has made a fatal error in choosing to let Mark Antony live and speak at Caesar's funeral.  The course of events will soon sadly reveal that Brutus' judgment in the matter of the battle against Philippi is also flawed, and will lead to disaster.