I played FFIII and FFIV in Nintendo DS, and imo FFIV is more hard than III. The last cave is awful and you need a lot of grind if you don't wanna to become overwhelmed against 2 or 4 mages pack.

FFIII can be frustrating if you move away from the right path because mobs can kill you easy.
Btw, i think the III jobs is fantastic, differents jobs and possibilities to create your group.

Per example, white mage and devout. The first one has a lot low level spells but not powerfull. Devout, by contrast, has a few low, mid and high rank, but that high level spells are powerful.

To leveling jobs, you didn't need fight against high mobs, just search a low mobs in a cave, battle and guard around 5-15 times and then end the fight. Using jobs abilities increase the xp earn