PSX EP doesn't have PSP IS's horrible encounter rate, for one. The battle system is also sort of different. In EP you set up gambits and let them play out, monitoring battles to make adjustments as necessary. In IS PSP you must pass through a hundred billion menus just to set up your turn, and if you don't manually disable like, half the things in battle combat turns take forever. While ISP has an advantage in that it informs you whose turn will occur when, a minimap for some navigational ish, and bonus content, EPX's smoother running battles, fewer encounters, and overall playstyle just left it feeling less tedious to me.
IS PSP has better cosmetics, of course. ISP's voice actors are much, much better than EPX's, for instance.