The problem with the statement "they r built on the same platform" is that the FX, M, and I think EX are all built on the same platform with a variant of the same VQ35.
All Front Mid-ship. All RWD. All with the VQ35DE or VQ35HR. But if you said the Z33 and the FX were the same car, people would laugh at you.
Nissan ran into some serious problems back in the late 90s and early 2000s when the American public realized that most of the Infiniti line were just upgraded versions of the Nissan line (Maxima=I-series, G20-Sentra, QX4=Pathfinder). Around 02/03 they made a change to have the two separate lines of vehicles to avoid the same issue.
So even though, they share platforms, the body styling, and seating configuration were kept different. My understanding is that the Infiniti line still holds slightly better quality too (ergo the super extended warranty).