Hmmm... That's actually a fairly close call, but you could get by going either way and up to moderate loud, the PB12-NSD wouldn't be stressing at all. The PB12-Plus would let you go up to very loud levels in a room your size. Not to mention to custom tuning ability and the improved bass quality of the Plus.
I don't think you could be disappointed going with either choice.
subwoofers from SVSound are pretty much as advertised. In other words, having owned several subs from them, the next level sub up in price and performance, is exactly that- a true upgrade. That isn't always true with other companies. I would go with the largest sub you can afford (with your WAF, of course). Dennis