Which resonance forms of silicon dioxide contribute the most to the actual bonding?

Sulfur dioxide, or SO2, has two resonance structures which contribute equally to the overall hybrid structure of the molecule. However, a third Lewis structure can be drawn for SO2 which is more stable in theory, but doesn't quite match experimental data. All 18 valence electrons are accounted for - 6 electrons from 3 bonds and 10 electrons distributed as lone pairs on the three atoms. A couple of important things to notice here. These two resonance structures are equivalent and will contribute equally to the hybrid structure. Both structures have formal charges - the negative formal charge is placed on the more electronegative atom - oxygen, while the positive charge is placed on sulfur, the less electronegative of the two atoms.