Visually, the three systems vary. Technically, they should be the same, i.e. each class has 10 different weapons (or maybe only 9 or 8 to leave the keys 0 and maybe 9 free for other purposes, but in that case it will still apply to all classes the same), and each weapon number has a very definite type of spell attached to it (e.g. weapon nr. 2 for the fighter is always a one-handed axe, weapon nr. 6 for the mage is always a fireball type of spell etc).
For example, if the fighter finds or buys a new, better axe and equips it, it modifies the attributes of the nr. 2 weapon and maybe replaces the sprites too (depending on the type of difference it may or may not differ, or if we are short on resources it may always stay the same), but technically it's the same weapon, i.e. a one handed axe.
In the case of the mage, his spellbook has an ever growing number of spells, but they are automatically organized into main categories (offensive, non-offensive, wands, staff enchants) and into subcategories (fireball, arcane projectiles etc). If the mage decides to memorize an offensive spell, its subcategory/type automatically determines the attributes of which weapon slot are getting overwritten. Whichever spell of a subcategory/type the mage memorized last is the one that is active on that key, if he wants any other spell to be castable he needs to "forget" the one he learned and memorize the desired new one.
For the cleric, the system is once again different (scrolls in a scrollcase), and there is also the issue of weapon upgrades (e.g. adding stronger sinew to a crossbow so it does more damage or fires faster), but I'd leave those things out for the first beta, just let's implement the weapon replacement scenario for the fighter and the spell category system for the mage, both with only 2-3 weapon slots.