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“better” as in “more reliable” wouldn’t matter if the evidence provided by both sources agree – it is only an issue if they conflict/contradict.
An auditor should always be alert to any audit evidence which contradicts other audit evidence (professional scepticism) and would have to weigh up the evidence on both sides (professional judgment) to reach a conclusion. So even if internal oral evidence contradicts external written evidence the latter does not automatically override the former.
That most audit evidence is “persuasive rather than conclusive” is one of the inherent limitations of an audit – which is why assurance is only reasonable and not absolute.