A 1179

Spontaneous casting only affect casting time when using metamagic. Metamagic used by a spontaneous caster always takes at least a full round action i.e. no Quickened spells for spontaneous casters.

Otherwise the difference is in spell preparation or lack thereof. A preparing caster needs to decide which spell goes into which spell slot every day. A spontaneous caster does not. He can simply decide which spell to use. He is only limited by his spells per day.

Example:
Both casters know only Magic missile and Shield. Both get 3 level 1 spells per day.

A preparing caster would have to decide how often to prepare both spells. 2 magic missliles, 1 shield, or 3 magic missiles or whatever.

A spontaneous caster could decide spontaneously for which spell to use each of his spells per day