SVS seemed to sum it up pretty well on their site :
- cylinders are a bit lighter and less expensive to build for a given cabinet size, are easier to move around and take up a lot less floor space
- box subs are generally considered to be better looking, SEEM smaller for a given cabinet size, and are easier to finish in a way that fits nicely into a living room. It's also kind of hard to make a cylinder sub with multiple drivers
Since tuning for low frequencies seems to need long ports (I forget the math), I think it was easier to fit the required port tubes into a cylinder design (because they are tall and skinny, not because they are round). If you pull apart an SVS cylinder the port tubes run almost the full height of the box, and if you look at the Hsu turbo option the ports snake out the back and fold around in order to fit in a reasonable space.
One caution -- the cylinder look seems really sharp at first but you can get tired of it fairly quickly while box subs seem to be "timeless"
