I don't think anyone can really tell you how it will sound, really, but since you listen to techno and alternative, that helps with an answer. Sorry, I don't really know what alternative totally means, since it covers a huge range of music, but techno is easy.
The basics is that the sealed will allow you bass that ranges throughout the range of notes, from the highest bass notes down to the lowest rumbles. Ported will be able to play louder, but with a smaller note/sound range. So the higher and lower bass notes will not be as loud as the middle ones.
All JLs are great subs, so the choice is up to you. I would say, though that with Techno, you have super high frequencies, and only a few middle bass notes, since there's a lot of thump of the same note. In that case, probably a ported box would be better, because you'll never notice the difference with a sealed, since your music won't ever need to play those notes. If you want a box that will play a larger range of notes at the same sound level, then go with the sealed.