That's easy. Pick the "con" side; con wins two out of three times. That's because the very structure of debate favors the audacious attacker, no matter what the subject.
Pity the poor person who's job it is to defend what he thought was an unassailable eternal truth. Let's say he's defending the idea that we should love our mothers. As the attacker you casually ask why. The rattled defender, rattled because he's not used to hearing the idea questioned, awkwardly replies that mothers earn the love by doing so many favors for their children. "Oh, I see", says the attacker, "So in your opinion, favors buy love. Love is something that you can buy and sell, like apples." The defender winces and starts to wish that he were somewhere else.
The attacker takes out a dollar bill and asks; "Would you love me if I gave you a dollar?" The audience howls with derisive laughter. The debate's only just started and the defender's already lost.
But we all know that loving your mother is a good thing, whether we can prove it in debate or not. So why is debate such a horrible way to discuss some issues?
It seems to me that debate fails because it gives too much weight to the attacker. It's just too easy to make the other guy look like a monkey, especially if he's not as funny as you, and not as inclined to exaggerate common ideas til they sound crazy.
Add to that the fact that the defender's burdened with defending every real-world decision his people ever made, going back to the days before he was born. The attacker, on the other hand, can argue from a Utopian idea that's never been tried, and therefore has never been found wanting. In debate both views have equal weight.
Of course sometimes attackers are right, which is one reason that we still need debates.
