Very different characters, I think. Both are noble Romans - high-ranking politicians. Both have military experience. And it's about there that the similarities end.
Cassius is, I think, the single most perceptive character in the play. He always gets it right: he knows exactly how to tempt Brutus into the conspiracy by playing to Brutus' self-important idea of his own honour and nobility, he knows that Casca is much more than Brutus thinks him ("a blunt fellow") and that he will be of use to the conspiracy, he argues that Antony should be killed alongside Caesar (in retrospect, a very good idea!) and - finally - the tactics that he suggests to Brutus for the Battle of Philippi are proved right in the event.
Unfortunately, Cassius is always over-ridden (usually by Brutus). He is mocked ("lean and hungry look") by Caesar (it seems, in front of all sorts of his colleagues) who knows his danger. People don't seem to respect or trust him as they do Brutus - though he is by far the better statesman.
(more in the next post)
Antony, like Cassius, is underestimated by other characters. For the first half of the play, Shakespeare makes it clear that he's Caesar's right-hand man - and that he enjoys masques and revels. Brutus and Cassius are scornful about him. Yet he alone seizes the political moment after the assassination and turns the tide against the conspirators in his inspired speech at Caesar's funeral.
Unlike Cassius, he seems to deeply care for Caesar (in fact, Cassius doesn't really seem to care for anyone - where is Mrs. Cassius? Never mentioned). Yet he's not simply the golden boy. He's actually a very, very canny politician. How, for example, does he know where Brutus' and Cassius' daggers ran through Caesar's mantle - he's not in the scene with the murder? And, though he has the will at the funeral, he sends Lepidus to get it in the next scene in which he appears, Act 4, Scene 1:
But, Lepidus, go you to Caesar's house,
Fetch the will hither, and we shall determine
How to cut off some charge in legacies.
Read carefully. Antony wants to cut down Caesar's legacies - why? Is it to accommodate the promises he made at the funeral? Was Antony's "will" a fake? It's never answered.
Note too that Antony, who in his funeral speech hammers home the point that Brutus is dishonourable, finishes the play by calling him "the noblest Roman of them all". Like Cassius, Antony is a canny political contriver.
Antony and Cassius are of different social classes which means no matter how high each rise within the government, Antony would always be reminded of his status.
Are you sure? I don't think there's any evidence in Shakespeare's play (which, remember is not based particularly strongly on history - think of the chiming clock in the orchard scene!) to support this: though if you've got some I'd love to hear it.
In terms of social status, both Antony and Cassius are high-ranking officials in the Roman government. Both are the right-hand man of the two primary characters in the play--Caesar (Antony) and Brutus (Cassius). Both are also very intelligent and shrewd characters, manipulating others throughout the course of the play. Cassius is the catalyst that begins both the downfall of Caesar and Brutus. He is the gnat, whispering in Brutus' ear, much like Iago is to Othello to mention another Shakespearean play, convincing the loyal and patriotic Brutus that the best thing for Rome is to assasinate Caesar before he becomes a tyrant. However, Cassius' reasons for dethroning Ceasar or more personal and vengeful than he leads Brutus to believe. Likewise, Antony is also highly manipulative. He convinces the rabid crowd through his famous "Friends, Romans, and Countrymen" speech to avenge Caesar's death. Antony seems to become more self-serving as the play progresses, much like the man he maligns, Cassius. Towards the end, he has formed an alliance with Octavius and Lepidus, and proposes a list of Romans who should be killed. Like Cassius, Antony fights the war not for Rome, but for his own personal need for vengeance against the conspirators.
It might seem as if Antony and Cassius both have a common agenda, however one must understand the mindset of each of these men. Antony and Cassius are of different social classes which means no matter how high each rise within the government, Antony would always be reminded of his status. Aside from that Antony could never manipulate Caesar the way Cassius was able to manipulate Brutus simply because Caesar was Caesar. Julius Caesar recognized the loyalty of Antony, a feeling Brutus never felt for Cassius. Long before Caesar was murdered, Cassius' intent solely rest upon juxaposing Brutus (and his ultimate actions) against saving the Roman Republic. Antony never possessed any such intent however, after Caesar's assassination Antony does try to avail himself to promoting the 'right' Roman replacement....himself. There is no doubting these individuals' agendas, however it is how these different agendas ultimately manifest themselves that differentiates them from eachother. Cassius tried to monopolize his agenda by manipulating Brutus, Antony never made Caesar do anything...he simply waited for his turn, politically speaking there is a 'sacred' difference.