I have a PC13-Ultra (similar to the PB13-Ultra), and recently added an SB13-Ultra. I auditioned the SB13 by itself in the same location as my PC13 normally sits, and they were very similar (I was part of a contest here and wrote a review on it). My room is bigger than yours...but I don't listen to things very loudly most of the time. Both are great subs. If I had to pick one, I'd pick the SB13 simply because it's so much smaller but performs almost identically for my needs.
For shear volume, the PB13-U is going to win. But, whether you'll ever notice the difference depends on how loud you play things. My main recommendation - really consider the size and weight of the two choices.