I think you would do good to keep it simple and go with a sealed box. It provides good sound quality and are simple and inexpensive to make. I would sugest you NOT get a bandpass box. Even though they can make a sub PUOND
they don't provide as 'good' sound quality. I've found most to be just loud. Also it is harder to hear the sub start to distort if you are running them hard. Almost all the kids that came back to me with blown subs either had bandpass or didn't understand the first thing about speakers.
As far as the advantage of running a 15W6 what are you comparing it to a 15W3? Advantage over what?