That system sounds pretty insane. Boxes are preferance, and it also depends on the style of music you enjoy.
A sealed box will create more of a quick, loud, hard hit which will reverberate extremely, causing more car vibration than a ported box.
A ported box creates more of a sustained hit. The tone drags out a little bit longer, and the bass is more full and deep. The bass is more distinguished when the subwoofer is working less, since it's easier to produce noticable sound with a ported box.
In reality, it's not a huge, huge difference, especially if the subwoofer is hitting from behind seats. It depends on what you like. To me, I get better listening pleasure out of a ported box because the sound kind of cleaner and more full. As far as impressing people that you drive past; a sealed box will probably create a bit more of an insta-headache smack.