Perhaps a combination, like in Ars Magica? Spontaneous casting gives the user great flexibility against unknown challenges, but with the drawback that a quick-and-dirty solution isn't necessarily going to be perfect. Vancian ritual spells are assumed to be built from the same components as spontaneous magic, but pre-constructed for a specific purpose, tested, and utterly reliable—if rigid in application.

Doing this could unify the two primary arcane casting classes. Sorcery is defined as spontaneous casting, while wizardry is ritual magic. Your "class" is merely fluff to define which you do more often.