It's kind of difficult to say really, cause on the one hand, the DS version really beefed up a lot of the monsters and bosses but you also have the augment system which allows you to overcome it a bit better. In the PSX/Hardtype version, it's noticeably more challenging than the SNES version and getting a party wipe from random monsters is not uncommon in the game, yet strategies and set-ups that worked in Easy Type still work so it's not like the game isn't going to throw anything at you that you haven't seen. You'll just notice that the game likes to nail you with debilitating status effects that tend to have high success rates, and certain attacks suddenly do close to 2x or sometimes rarely 3x damage to your party. Zeromus and many of the optional bosses in the Lunar Core are far nastier than you would remember. The GBA version emulates the original hardtype to a certain point but I never felt it had the same level of difficulty spikes in the later sections of the game like the PSX version did.

Overall, I might say the DS version is definetly the harder version, simply cause it involves having to relearn a lot of the bosses and enemies. Golbez and the CPU bosses are both notoriously more difficult in the game if you are not prepared, though some bosses (like Zermous) are actually much easier since he isn't just spamming his party rape spell Big Bang over and over again, not to mention they removed the whole trick with the Dark Matter item which is what made that fight challenging in the original difficulty.