If you have the room I would go for the PB2+ it has the better drivers more watts and can be tuned down to 16hz! I do have to warn you this thing is big!
I went for two PB1-Plus units and I am very pleased with the results. It depends on your room size & placement options, whether one or two (or even more) Subs are advisable.
I have looked at this question too. My personal conclusion is that it you're looking for raw output, two PB1-ISD's might be better. But if you're looking for low freq extension, one PB2+ is probably better. The reason why is that even though driver area is the same, the PB1-ISD low freq extension is still limited to 20 - 22 Hz, even though you have two of them. They just play louder. Don't go any deeper. But for the PB2+, the two drivers are equalized together in the box for low freq extension too. But then again, the PB2+ can be "tuned" to different low freqs (and outputs) with those muffler things. So ultimately the single PB2+ also gives you more flexibility too. But, ... 2 PB1-ISD's are easier on your back!