Personally, if I were to do away with the Scribing cost for Wizards, I would allow the Sorcerer bonus spells known.
What I have used in the past as a balance factor is that the Sorcerer gains anumber of bonus spells known per spell level based on Wisdom.
Yes Wisdom. Why?
2 Reasons:
1) Mech Balance: Most of the classes are designed in such a way as to make the Class rely on more than one single primary stat.
Example: Cleric - Spells based on Wisdom, Turning Based on Charisma.
2) Fluff: Per the description of the Sorcerer, they are supposed to learn their spells as Innate ability - they just "develop" in the Sorcerer. They do not study spell books the way wizards do to learn spells. If they use charisma as the force of will to cast the spells, then the innate knowledge of what they can cast should come from Wisdom. It could be argued to use Intelligence - but it depends on your view of how they learn spells.
Just my $0.02 worth.