Looking over the SNS option, it seems there are more ways to customize it. I'm sure talking to Jan you could choose the size of the bottom, the size of the bag then the size of the top. With PRB - they pretty much said what they have is what you get.
As mentioned, I have the Large PRB for my 15'6"aire and it works fine. It takes some finesse to get it to bear the load on the frame and tubes and the bottom of the bag is just a hair too large for the space I have. Small would be too small. As mentioned earlier, I would like to get a medium base, large sides and large top. I would imagine that SNS could actually do this.
"It's supposed to be hard. If it wasn't hard, everyone would do it. The hard... is what makes it great."