Wood or Metal
The most noticable difference between a wood or metal bat is the sounds that each create when struck by the ball. I'm quite partial to the sound of a wood bat myself. These differences occur because of the different vibrations that occur in each material. A wood bat has fewer vibration waves than a metal one. This gives the metal bat a greater 'sweet spot,' but it also causes a greater sting if you miss that sweet spot. So what would be a give and take for the usage of metal bats have been changed thanks to newer technology. New metal bats have certain qualities that help dampen the sting from a miss-hit ball.
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The 'ping' sound made from a metal bat has also been a sign of the "trampoline effect." This effect is thought to allow an aluminum bat to 'bounce' the ball further and quicker off the bat. When compared to a wooden bat, there is little difference if both bats hit the ball at their sweet spots. The "effect" from metal bat comes into play when you miss the sweet spot. When you're off a little with a wooden bat, the bat takes away from the balls kinetic energy resulting in a weaker hit. With a metal bat the bending vibrations still take away from the energy but the cylnder vibrations return that enery back to the ball. There are times when the rebounded energy can be greater than the energy originally taken away from the ball, causeing the ball to fly off the bat much quicker than the pitcher had delivered it. Which raises many safety issues as to whether or not metal bats should be allowed to be used in lower level baseball leagues.