The stats are usually randomized by a couple points. Stats can be altered through materia use though. Basically everyone is the same. What makes them different is the weapons they use and their limit breaks.

Want a real challenge i say use any one character, keep their first weapon equiped, don't use limits and don't equip materia. Should be challenging and the use of no materia equipped can help give a high HP by the end of the game, so you could consider that a reward, i guess. More use of item magics and such would also give you a different type of fighting experience.