The magic damage stat on a weapon is the magic-based damage the weapon itself inflicts. The damage is counted alongside melee (or ranged, if its a bow). It will have no effect on your spellcasting.
In that case, the Cleaver is definitely better, unless you plan to enchant them with Increase Damage enchants. In that case, the Slayerbane is better, due to two slots (Increase Damage and Increase Magical Damage can both be put on it).
And yeah, Noryfundus' stuff, earrings (and potions) aside, are all fake. When you buy them, as soon as you leave the trade screen, they revert to their useless version.
