I went with the cylinder (20-39), primarily because of the guidance provided by SVS on their site. At the time (and probably today), they presented the cylinder as being slightly higher performance at the respective price point, mainly due to the simpler job of making a cylinder-shaped sub structurally sufficient. The lower cost of development and production were passed on to the buyer.
Esthetics are a significant factor. If you prefer (big) furniture, the box will likely appeal to you more. The cylinder has its own appeal, in my (biased) opinion, but you won't find any pretty wood veneer covering it.
The good news? Whichever you choose, when you put that movie or music track on, you'd better hold on.