High level gear usually falls into 7 sets(casting, healing, fending, aiming, scouting, striking, maiming) that the 15 classes are split up into. But just because 2+ classes share a set doesnt mean that they arent different. Black mage and red mage are both magic dps classes. They can share most(~95%) equipment. They dont play alike. Black mage is built around getting into the offensive stance and pushing out as much damage as possible as fast as possible, then switching to a regen stance to build your strength back up. They have several "long" cast spells that they use via hard-cast regularly, and usually have to stand still to get a spell off. They have no unique party utility spells. Their most important stat is speed(faster casts means more damage and more chances to move without interruption, they already have very hard hitting spells so crit/direct is a secondary concern). Red mage is built around balancing two mana pools, balancing melee combos versus big spells, balancing quick spells with long spells via class skills, and balancing damage and utility abilities. They want to use a quick spell to trigger an instant cast for a longer harder hitting spell to follow, and since every other spell should be instant they have a lot of chances to move and still finish a cast. At high levels they can heal, res, and buff comrades giving them many utility spells(instant resing with the only limit being their mana pool is very handy when trying new content). Their least important stat is speed(instant doesnt get faster via a speed boost, and their spells are a little weaker so they want the direct/crit boost much more).

Hopefully this makes more sense.