Pros of an aluminum bat:
The are less likely to break
The "sweet spot" is greater, thus leaving more room for error.
They are lighter, thus generating more bat speed which results in hitting the ball harder.
They are metal and you will get more "pop" off of it regaurdless of size.

Cons of an aluminum bat:
As players get stronger and pitchers throw harder the ball jumps off the bat faster, more injuries. Very common in College baseball.
Batters can rely on the bigger sweet spot and not really develope a good swing.
Can not really get a good reading if the hitter is really good or really lucky.

Pros of a wood bat:
Teaches how to center the ball on the bat, less of a "sweet spot".
Promotes a shorter swing, a long swing might result in a broken bat.
Helps hitters understand the importance of bat speed.
The sound of wood is better than metal.

Cons of a wood bat:
They break.
Takes more bat speed and strength to generate distance.

It is not the size of the bat that matters, it is about how fast you can get it through the strike zone and how fast it is at impact with the ball.

Wooden and alluminum bats that are heavy hit the ball farther; whereas, light bats (doesn't matter which kind) hit the ball shorter.