All the detectors mentioned do exceptional in the environment they were designed for. The Fisher Gold Bug, and the Whites gold units were designed for gold nuggets in the gold fields. The Headhunter PI was designed to find gold jewelry in the salt water. The Fisher would do poorly in the salt, and the Headhunter would do poorly in the nugget fields.
Gold jewelry, and gold nuggets are not the same type of targets, and their conductivity is different.
Mr. Bill