I tried a sealed box and a ported box in my 330I. I found that with the seats up the sealed box hit harder, but was not loud. Even with the seats down they were not doing what i wanted. I had to also max out my amp to drive the sub. The ported one was not to bad with the seats up, however if u drop the seats it sounded awesome. If had the seats up, what i would do sometimes with the ported box was drop the arm rest in the middle. That was the only way to get anything through those metal seats. lol. Using this setup also allowed me to turn down the amp and not drive it so hard.
It also depends on what kinda of music u like, i would say hip hop/rap go ported. Rock/ jazz go sealed. However if u are looking of SQ then i would say first find the driver u want and build the box to its specs.
I hoped this helped. I am sure some of the other guys could give u a more technical answer.