The short answer is "I don't know."

Supposedly you're able to get "incredibly rare" weapons or some such, but I've only done Easy and Normal and they have similar weapons to what you find in the game. Well, they might be a bit more powerful or have more added effects.
I guess it's possible that the Hell dungeon might have "ultimate" weapons - but I haven't even cleared the Hard dungeon so I honestly can't say.
But personally, I think the whole purpose of the dungeon is just to appeal to the hard core players who want something "more" out of the game - here is an honest-to-goodness endless, random, unlimited dungeon crawler, inside a Kemco RPG.
Perhaps someone else has more information on this?