I noticed that the PB13-Ultra seems to be much more common than the PC version and I wonder why that is? The SVS site states that the PC is more efficient and more cost effective design, other than personal preference of the different form factors, is there a reason most people seem to gone with the PB over the PC?
People like to waste money.
It is the form factor that some people don't like, and that is the reason for the price difference as well. Cylinders give a better bang for your buck, so if that is the main consideration, one will go with a cylinder. If one prefers to waste money, one will select a box.
The cylinders take up less floor space, but are taller. So which will be better for a particular person might be determined by that.
I personally don't think a huge box is all that attractive in a room, and don't see why anyone would think that a rectangular solid shape is more (or less) inherently attractive than a cylindrical shape, but there is no accounting for the taste of others.
Here is what SVS says about it:
http://www.svsound.com/questions-faqs.cfm#cylinderorbox