Bandpass sucks. Especially pre fab. Yes they may seam louder, but that is because they are not designed for the speaker, so the speaker starts to distort. Stick with sealed (especially if going with a pre fab), or have a custom box built with a slot port. Make sure they tune it to match your speakers.
bandpass enclosures are more efficient, usually adding +3 to +6dB in SPL over a sealed box, but they are larger, usually about 1.5X the size of a sealed box or larger. as the name implies, the bandpass box is limited in it's frequency response, and thus usually will not play very low bass. It's peak efficiency is usually in the range of 40-70Hz, with steep rolloff beyond those points. Bandpass boxes have the ability to mask distortion which helps some in SQ, but only within the range the box is designed to play. Bandpass boxes need to be tuned accurately for the specific sub they contain, and failure to do so can easily damage the driver.
generally, bandpass boxes are good for SPL events, and rap music, but not the greatest for a sound quality system due to their limited response range and lack of low end extension.