The FiCar woofers will, for sure, outperform the Maelstrom-X's simply because you are comparing two to four. SPL, flat out will be higher with the FiCar's. Really one can't say that the SQ will be much different between the two at nominal levels. Only until you push them to their extremes will any difference in SQ really be detectable.
I'd say that the IB will be the one to go for. There are caveats though. With dual M-X's you can at least have some flexibility in room placement. With an IB it's done and done with. If it sounds like crap in that spot you won't have much luck fixing any FR irregularities. Also, some EQ may be needed for a desired frequency response. The two Maelstrom's on the other hand will have a very natural droop (or "shelf" depending on the alignment) to it's lower limit.
So if you haven't already, at least get a measurement or two of a sub in the spot where the IB will go and see if it's okay at the listening position. If all looks good from there.... then go build it. You'll love it.
Oh, and I'd love two Maelstrom's actually.
(nah, I can't afford them atm so I'm cool for now.