If you go with dual (considering the size of your room and that is sealed), then you will be more than fine with two PB12-Plus. Thinking in your room with dual Ultras it will be just too much..........but hey, they are other fellas that will do that no matter what anyway.....
It will not be just the output, but as you already know the bass will be more smooth running dual subs in your room. So, if I was you having to pick a single Ultra........vs. two Pluses.......I will go with the two PB12-Plus.
Again, worst case scenario, you can upgrade to "two" Ultras later........if you think your drywall nails are not pupping off as quick as you want......LOL.
So, my vote is for two PB12-Plus.
What AV Receiver do you have?
I guess may be a single PB12+ for now. IF, a big one, that's not enough, I'll probably get another one down the road.