I would say "fresh eyes". My experience with a cross-functional internal audit team is that non-quality members are comparatively less familiar with the QMS and less skillful. However, their professional background and the naiveness allow them to see what a quality member may not see or take as "normal" and to ask good questions. For example, a design engineer may be a great resource for a validation process audit. The audit may also inspire the auditors to better do their own job.
"More resource"? Maybe, at least at the beginning. With my team, within 2 years, we lost all engineer internal auditors (who all volunteered to become an auditor and took training) to engineering managers who could no longer afford resource loss. To compensate, we now require all Quality personnel to be internal auditors.