Hi. I recommend that you contact SVS and ask them. They are very good about recommending what they think the best sub for the job is.
The Ultra is a great sub, but the PB12-Plus is no slouch. You could always add a second sub in the future (a) if you want to and (b) if/when you get the money to afford it. People here, including the founder (Gene), have discussed the benefits of having multiple subs instead of just one.
BTW, since you are already ordering from them, I believe that you'll get a discount on the sub.