I'm gonna buck the trend and claim that the Wizard would make a better prestige class than base class.
Why? There spellcasting ability is *so* general and powerful that even at low levels they feel like the pinnacle of magical ability. They can learn an unlimited number of spells, and research any spell they want. Their only limitations are spell slots and gold. The ability to do anything one can imagine, given time, just feels like it should be the pinnacle of one's career rather than the beginning.
Even the Eberron setting recognizes wizards as something rare and unusual for this.
I have toyed with the idea of a setting where the only base classes are either mundane
or grant limited and specific magical abilities. None of the base classes would be able to
prepare or research spells. Paladins (without spells) and monks would be OK, and
probably the only spellcasting class available would be the sorcerer.
All the classes that prepare spells would then become prestige classes, with the prerequisite that you already possess at least 3 supernatural or spell-like abilities.
The basic idea is that if you want to learn how to take advantage of magic's infinite flexibility, you had better have some innate magic to begin with.
Ben