Why not just use the sorceror since that is what the spontaneous caster is judged against.
Your "wizard" would still blow the doors off the standard sorceror.
In your system, why would a wizard NOT leave all their slots open? Sure, they only get so many castings per day, but given that they will ALWAYS have the best spell at hand, they don't really need that much unlike a sorceror who has to use fireball as a signal flare, damage spell and a door knocker.