I beleive the general consensus is sealed is better and the Fathom F113 is quite possibly the best one out there.
The concensus in the DIY community, if there is one, is that large ported is the best. The problem with sealed is that you need EQ and a LOT of power to get a sealed sub to extend really low. That usually introduces distortion.
For example, the MaelstormX 18" driver, because of its high linearity, is very well suited either to a 22-24cu.ft. ported enclosure tuned to 12 Hz, or a sealed enclosure.
Ported, it will have a -3db point of around 12Hz (lower in room), and will hit 113 db at 12Hz with 500w of power applied.
Sealed it will have a -3db point of 30Hz. at 20Hz it will be -8db and at 12 Hz -16db. In order to flatten the curve, you will need to apply EQ, which very quickly introduces distortion. Also, as the driver needs to move more to displace more air for lower frequencies, it needs more power to overcome the natural damping of the sealed enclosure.
The long and short of it being, you get more out of a good driver in a ported enclosure.
