Sealed subs tend to give a tighter/snappier sound than ported boxes at the expense of being less efficient. They also can go lower given the same driver being used. Sealed boxes roll off at 12 dB per octave while poreted boxes are 24 dB per octave. So the roll-off on the sealed is much shallower. Ported boxes are louder all the way to its tuning frequency. A filter should be used in a ported sub so that it doesn't play much below it's tuning frequency. This would cause the cone to flap around and possibly cause damage, because it became unloaded from the box (it's just like running the sub in free-air). They tend to be not as tight. and better for movies. However, well designed ported and sealed boxes can be just as good for both.

-Joey