Well, between Photoshop and Painter, I believe it just depends on what you are more comfortable using. Maybe they use Painter because they are more used to it in some aspects than Photoshop. As for Photoshop and Illustrator comparison, it's a matter of which software specializes in some functions/purposes like Illustration is better for vector-based works than Photoshop. In the end, it all depends on the artist and what he does.

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