Bryan,
A buddy of mine has the Fi 18" + the older EP2500 in a room about the same size as yours. Those things rock the house. Make sure when you build your manafold to fire the drivers at each other. His first version had them firing into the room. His ceiling joists were flexing like crazy from the force of the drivers. Once he changed them to fire into each other, no flexing or vibration.
I say definitely go with an IB over a ported/sealed sub design. The only way I can describe it is that the bass seems smoother and more natural -- not boomy. I have (4) SoundSplinter 15s in my 19 x 15 room and they rock. You feel the bass hit your entire body (no butt kickers needed in my room). When I bought mine, the 18s had not been released yet. If they had, I would have bought a pair of those and called it a day. You can't beat the price/performance ratio with IB and Fi.