Ported vs sealed is a can of worms (the debate rages) but here goes...
Ported have quite a bit more output, especially around their tuning frequency but they are also a bit bigger for the same size driver. They also have very low distortion around their tuning frequency. As such they are generally very good value for money and are great for home theatre and many enjoy them for music too. Some of the highest regarded subwoofer designs are ported.
Sealed have a more natural roll-off in the lowest frequencies. Fans of ported subs will flame me for this but - I prefer sealed subs due to what I perceive as a tighter, more accurate sound which is especially important for music. This is the effect of group delay which is generally lower in sealed subs. The following is an interesting article by Adire Audio about group delay...http://www.adireaudio.com/Files/GroupDelay.pdf
For bookshelf/monitor speakers, sealed subs have an added advantage in that they do not suffer from port resonance when crossed over higher.