"Otherworld" on Disk 1, is the recorded version of the song. Heard in the opening Zanarkand FMV of course.

The Otherworld played while fighting BFA is a sequenced psf2 version of it. In other words, it is just an Otherworld generated by the PS2 synthesiser hardware, meaning that the song can be looped, etc. It's like SPC, NSF, etc. It's the ORIGINAL version of the song (not some crappy MIDI).

If you want to get your hands on this version, you'll need a psf player, such as Highly Experimental for Winamp, and the actual FFX music files.