Just recently I was in the same boat as you trying to decide which SVS to purchase. I contacted SVS with the same 2 choices you have selected and SVS respond with this.
"Hey Michael;
Sorry for the very long delay in reply, we had some email issues and it
Looks like your note got held up. Remind me when you order and we'll
Include a t-shirt with your order as our way of saying sorry for the delay.
Personally I would get the PB12-Plus/2; it'll be more powerful (about 3-4 db
Above 25hz). The PB12-Ultra might have slightly better sound quality but it
Won’t be a night and day difference and I think the difference in output is
Worth the slight loss in sound quality.
Erik K"
And if you did not notice they have great customer service, by far the best online customer service I have ever dealt with. I guaranty you will be satisfied.