One of the most original arguments in the sculpture vs. painting debate came from Bernini, the Italian sculptor.
Gian Lorenzo Bernini, Self-Portrait [38 × 30 cm (14.96 × 11.81 in)]
Museo e Galleria Borghese, Rome (Wikipedia Commons photo)
He told the French Academy that the painter has this great advantage over the sculptor: he learns while he paints and can correct his work as he proceeds. His painting, therefore, is the result of all he has learned while painting it.
The sculptor, for his part, must stick with his original idea, since he cannot correct it. His statue, therefore, is a record of what he knew only before he started to carve.
That might have been true for Bernini, who carved his figures right off in marble, without solving all the problems first in a sketch. He hated to go to the trouble of finishing a model. “If I did the work in clay or wax first, then all I’d be doing in marble was copying myself. I want my marble to be the original work, not a copy.”
During his portrait sessions with the Sun King, for instance, he carved the marble bust directly.
Louis XIV (1665) by Bernini (H. 105 cm; W. 99 cm, Depth 46 cm); Diana Salon, Grand Apartment, Palace of Versailles. Creative Commons Genérica de Atribución/Compartir-Igual 2.5 photo
No other sculptor would dare work directly in stone for a portrait. They all used sittings to make sketches or models and then copied them later in marble back in their studio.
Most of the old stone statues, because of their complexity, were copies of models the sculptor had finished completely. Nowadays, however, sculptors prefer to carve their figures right off if they can, with reference to only simple sketches, just like Bernini.