The cylinder design will give you almost identical performance. It is a different shape, but the woofer does not care much whether it is in a rectangular box or in a tube. The reason the cylinder is so much lighter is because a curved surface is inherently stronger than a flat surface, so it can be thinner and lighter while having the same strength, without needing a bunch of internal bracing that also adds weight. This helps makes it cheaper to make, and cheaper to ship (which, since shipping is built into the cost you pay, this is part of the difference in price). It also is a much easier shape to grab onto, and since it is significantly lighter, it is much, much easier to move than the box version.
I personally would not even consider the box version, as it is a bad value compared with the cylinder version. I own a pair of SVS cylinder Ultras (an old version, not the current one), and love them. I am glad I have the cylinders. The boxes waste your money, gives you something more difficult to move, and takes up more floor space. Judging by the specification differences, the boxes likely have a slightly larger interior volume than the cylinders (the exterior volume is significantly greater, but much of that is taken up by thicker sides and internal bracing that is necessary for it to equal the strength and rigidity of the curve of the cylinder). So there is a very slight difference in performance. The other downside is that the cylinders are taller. Not much in the way of downsides compared with all of the upsides.
It turns out, though, that many people buy their gear by how it looks as much as (or more than) by the sound, and many people have a prejudice about the cylinders. So many people spend extra money. But in a darkened home theater, you will not see either one very well or at all.
Here you can read more about the cylinder version:
http://www.soundandvision.com/content/test-report-svs-pc-13-ultra-subwoofer
http://www.hometheaterhifi.com/subwoofers/subwoofers-reviews/a-secrets-subwoofer-review.html
Buy the cylinder version. It is the more practical, logical choice.