Hard and hero mode gives you MUCH better materials. Better possibility of silver materials in normal officers instead of bronze and gold for commanders (you need the gold ones for the last skills in the tree of every character). ALso better weapons.
Adventure mode scales the difficulty (you cant choose it) depending of which square of the nes map you are, and you will see gold materials you never see in story mode. The best thing to grind are helping high level Links, of course. 45-60 seems to be the best for grinding.
Cheers. The dream of quickly plowing through replays is dead, lol.