He is not so much of a writer as a reporter. <-- Not correct. Omit "of".
He is more of a reporter than a writer. <-- Correct
He is less of a writer than a reporter. <-- Not correct if you mean any reporter. If you mean a specific person, it would be correct, e.g. He is less of a writer than his older brother.
He is not so much a writer as a reporter. <-- Correct
He is less a writer than a reporter. <-- Have never seen this construction.