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Both take a lot of liberties with the source material. But Special is technically closer in that it uses every single character from the games, including the changing protagonists, time passes in between story arcs similar to the game timeline, and the story arcs usually follow the basic idea of the original plots but with more emphasis on the villain plotline.
The anime follows a strict formula involving Ash being the main character, no real time passes across the show, and it's all meant to focus on Ash's random adventures traveling to collect badges and compete in the league. So several characters they can't fit into that formula get cut, and plot or setting details involving the passage of time get cut, and the villain team plotlines get severely streamlined.
