It's going to depend on the subs.
I'm not aware of any drivers that have been used in the front and down firing positions. Until you have that you are comparing apples and oranges.
Two really good down firing subs would leave you expecting better performance than two not so good front firing subs and vise versa.
Unless you plan to have your subs play different frequencies, plan on getting identical subs. There is an article on the main page for setting up multiple subs.
Home Theater Multiple Subwoofer Set-Up & Calibration Guide | Audioholics
It always comes down to whatever you can afford with this.
I just looked and the PB12-NSD is $600 while the PB-2000 is $800.
I'll race you to the NSD.
EDIT: So much want:
Dual SB12-NSD
That right there is condo dweller bliss.