Ok, Here we go.........
A correctly tuned ported cabinet will almost always be louder, although they tend to be more difficult to build, and are typically larger than sealed cabinets..... ( Better for SPL type applications..)
However, I have heard some badass sealed cabinets, that SLAM.. And are typically more suited for SQ, and even SQL applications... I have a 15" Audiopipe in a small sealed box stuffed in the garage, just to have music in the garage when I'm working, and its very musical, but doesnt hit very hard... I have the twin to that 15 in the wifes trunk in a 3.25CuFt vent tuned to 33Hz, and it pounds ( well as hard as a POS audiopipe sub can pound).......
I cannot tell you which is better, it kinda depends on what you're looking for. Personally I like ported , and I tune a little higher for daily use, then remove that one and use a larger one for SPL... ( Borrowed the idea from SMD, saw it in one of his builds, and decided to try it out).......
Hope that helps some.....