My room is larger than that and their recommendation was the same. I already have a PB-10 though, so his second suggestion was to just get another one to stack on top of it.
You won't find the 20-39 plus lacking. Their cylinder subs are every bit as good as their box subs. Those are their bread and butter subs, and until about a year ago, cylinders were all that they offered. For the floor space, that's what I would go with in your case.