In general, metal bats will result in farther hits for several reasons. Aluminum and titanium alloy bats have bigger sweet spots than wooden bats. A batter is more likely to hit the ball in the spot on the bat that produces the best energy transfer to the ball (i.e., the sweet spot).

Aluminum bats of the same weight as a wooden bat can have a wider barrel, so the ball is more likely to be hit solidly, instead of popped up or driven down.

Metal bats have a distinct advantage at hitting the ball of the handle--they won't break off in the batter's hand. The ball can actually be driven pretty hard off the handle of a metal bat.

Lastly, aluminum bats tend to be more elastic in the return of energy to the ball.

In the US they sell composite bats. When you hit the ball it goes off the bats harder. My team has almost the only composite bats in Canada, and our league is thinking of banning them (in Canada) because the ball goes too hard and too far. I don't think they will though.