Shakespeare presents Cassius as a strong character but one who has many faults. He is greedy, miserly, cunning, and potentially treacherous, but certainly not weak. He boasts about his courage: 

For once upon a raw and gusty day,
The troubled Tiber chafing with her shores,
Said Caesaar to me "Dar'st thou, Cassius, now
Leap in with me into this angry flood,
And swim to yonder point?" Upon the word,
Accoutred as I was I plunged in,
And bade him follow. So indeed he did.
The torrent roared, and we did buffet it
With lusty sinews, throwing it aside,
And stemming it with hearts of controversy.
But ere we could arrive the point proposed,
Caesar cried "Help me, Cassius, or I sink!"
Ay, as Aeneas our great ancestor
Did from the flames of Troy upon his shoulder
The old Achises bear, so from the waves of Tiber
Did I the tired Caesar.     (Act 1.2)

A bit later Caesar himself says to Antony:

Let me have men about me that are fat,
Sleek-headed men, and such as sleep anights.
Yon Cassius has a lean and hungry look.
He thinks too much. Such men are dangerous.

And he continues:

He reads much,
He is a great observer, and he looks
Quite through the deeds of men. He loves no plays,
As thou dost, Antony; he hears no music.
Seldom he smiles, and smiles in such a sort
As if he mocked himself, and scorned his spirit
That could be moved to smile at anything.
Such men as he be never at heart's ease
Whiles they behold a greater than themselves,
And therefore are they very dangerous.

Even Caesar is afraid of Cassius. It is Cassius who is the originator and organizer of the assassination plot. Without Cassius, Caesar would not have been killed. He would have gone on to become king or emperor. Cassius is a very strong, determined, resourceful man. The only times he appears to show weakness are when he has differences of opinion or quarrels with Brutus. It has been observed that Brutus continually overrules him. This is especially to be observed in their quarrel in Brutus' tent in Act 4, Scene 2, during which Cassius repeatedly threatens to kill Brutus. The quarrel comes to a head when Brutus tells him:

There is no terror, Cassius, in your threats,
For I am armed so strong in honesty
That they pass by me as the idle wind,
Which I respect not.

After this, Cassius resorts to denials and appeals to their old friendship but not to any more threats throughout their meeting. It would appear that both these men are strong--but Brutus is the stronger. Cassius is handicapped by the fact that he knows he needed Brutus from the beginning. Brutus is liked and honored by everyone, whereas Cassius knows he is not liked or honored because he knows himself to be what he is, which is pretty much as Caesar described him to Mark Antony.

Men are strong if they have something strong to motivate them. It doesn't necessarily have to be anything good, but they need motivation. Cassius is motivated, as Shakespeare shows, by selfishness, greed, ambition, envy, and hatred. Brutus is motivated by patriotism and idealism. He also has a strong sense of family honor he feels obliged to uphold. The two men do not make good partners. The discord that erupted in Brutus' tent was bound to erupt sooner or later. It would have done so even if they had won the battle with Antony and Octavius at Philippi and had become join rulers of Rome. In such a case, it might have been Cassius who overcame Brutus, because Cassius could and would plot against Brutus, while Brutus is too noble to think of doing such a thing to Cassius.