I have the MOFO-12 and I would recommend spending a little extra for it. I got mine for $135 new from uscaraudio.com. You need at least 1000 watts to push it though, and a large ported box.
But to answer your question, it depends what amp you hook them to, because you could use a 1 ohm stable amp and the dual 2 ohm, and it would be hooked up at 1 ohm delivering all the power that amp can dish out. If you get an amp that is only 2 ohm stable (much more common) then you would need to buy the dual 4 ohm, because it will be wired down to 2 ohms.. This is assuming you are running just one sub. If you wanted to run two subs, you would either have to get an amp that is 1 ohm stable because 2 4 ohm dual voice coil subs wire down to 1 ohm if they are put into 1 channel on the amp.
My mofo-12 takes all the juice my electrical system can dish out, so I would need to get h o alt and better battery and all if i wanted to run a second mofo. Overall I am very happy with it. It slams harder than all three of my old audiobahns did with the same amp.
I know about those power acoustik subs, if you give me more info I can give you better advice...
Hey thanks for your advice. I have a LT/980 2 channel Power Acoustik amp. I am trying to determine whether or not to sell old FUBR-12 or new FUBAR-12 or somohow hook up both.
You could hook both up, as long as each one had a separate box or separate chamber in the box. But those subs are not going to satisfy you for very long. I have found it is better to do things right the first time. Even if you have to wait another month or more to do it. I would sell all your gear on ebay or to friends for as much as u can, then get 2 alpine type R 12's and a decent amp to push them, then you will be satisfied with what you got, instead of just using the fubars for a while, wishing you had more bass...