Is Brutus a better leader than Cassius? Why?

The answer to this question is somewhat subjective (both characters are good leaders in their way), but, for me, a definitive answer can be found in Antony's final speech in Act 5, Scene 5, during which he comments on the defeated Brutus:

This was the noblest Roman of them all.

All the conspirators save only he

Did that they did in envy of great Caesar; 

He only in a general honest thought

And common good to all made one of them.

His life was gentle; and the elements

So mix'd in him that Nature might stand up

And say to all the world 'This was a man!' (68-75)

In this speech, Antony reflects on the fact that, out of all the conspirators that assassinated Caesar, only Brutus wanted to protect Rome and preserve the republic. All the other conspirators, including Cassius, killed Caesar because they envied his power. Brutus, however, was motivated by the altruistic desire to protect Roman citizens from a dictatorship. In light of Brutus' marked nobility and selflessness, it could be said that he was a better leader than Cassius.

Now, it is possible to argue against this standpoint. For example, while Brutus is the more noble of the two, Cassius is a more practical leader. Though Brutus somewhat naively assumes that Antony will not turn against the conspirators, Cassius suggests that they murder Antony along with Caesar in order to prevent an uprising. Following Brutus' advice, the conspirators do not kill Antony, and Antony inspires a rising against the conspirators that leads to their deaths. As such, Cassius shows himself to be more practical, and this quality could lead some to assume that he is the better leader. 

However, at the end of the day, I generally consider Brutus to be the better leader, as I believe his courage and selflessness are superior to Cassius' practicality. The conspirators need the honorable Brutus to make their schemes plausible, and it is ultimately the nobility of Brutus that lends the play its tragic poetry. As such, I believe it's obvious that Brutus is the better leader.