There's a major difference between the PSP version and the older ones, namely the translation and art were improved. The class mechanics were completely re-hauled, removing any semblance of character building and replacing it with a more homogenized system. Instead of individual character levels, each class levels up. It also inserted an extremely grindy crafting system, complete with crafting failure chance. I put over 100 hours into that game, most of it crafting items. Each version is good, but I prefer the PS1 version over the rest.