These are built-in "windows". That is - they do not open, there is no casement. The exterior frame is rotting. Should the replacement be wood or a synthetic like duraboard. Is there a big cost difference?
To answer your question I would definitely use wood, that's how I get all my carpenters to fix a frame, wood is best. There is a significant price difference for using wood, none the less I would strictly use wood, I would then clad it with aluminum cladding.
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