It would take roughly 3x SB12s to match the PB12 in output from 16hz to 30hz. Above that, the SB12 and PB12 are pretty much the same.
The SB12 however will continue to have output down to 12hz where as the PB12 just dies off after its port tune (17hz). So depending on your room size, you may still get some infrasonic benefits due to room gain especially with 2 SB12s over a single PB12.
I'd always take 2 subs over 1 sub to be honest. 2 SB12s while it would not even give you more output than the PB12 below 30hz, you would however benefit from a much better in-room response. That's a lot more important than just output really, unless you're in a 6,000 cu. ft room or even bigger where room modes then start to become less of an issue.
SVS Ultra Bookshelf + SVS Ultra Center
Prime Bookshelf Surround with Prime Elevation Heights
Dual Rythmik FV15HP
Yamaha RX-A3060 + Parasound Halo A31 + Emotiva A500