Don't forget that there is the original Square releases and the Sunsoft re-releases of each of the 4 titles. I'm sure the Sunsoft versions are cheaper, though I did sell my CIB Square version of FFLII for 8.50...
If I were to be collecting every US FF release. I would include everything that has Final Fantasy on the box. Don't forget about Crisis Core when that gets released on PSP.
The only ones I've really felt have been redundant ports, have been the PC versions of Final Fantasy VII and VIII, as well as the PC and Xbox 360 versions of Final Fantasy XI. They're all fun to play, but I don't feel the need to own those versions as well as their original PS counterparts.
All the other "ports" to PS1, PSP, DS etc have all had plenty of original content to warrant being collected alongside the original version. New opening FMVs, redone graphics or music, etc.
all IMO of couse.