Its harder to save versus a Cloud Kill than a Sleep spell. Bards and Sorcerers use Charisma to cast their spells and don't have to memorize them. They can choose to cast any spell from their spell list at the time of casting. This comes from the fact that their spellcasting is more innate and less learned. Due to this fact, they have fewer spells in their spell lists, however.
Clerics are much cooler.
They can trade in uncast spell levels for dice of healing. Cleric turning is based on Charisma. All spell casters get bonus spells based on high abilities. Humans and Fighters get extra Feats to make up for their lack of racial or class abilibies. Mages get bonus Meta-Magic or Item Creation Feats.