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Pinpoint better so less damage by digging smaller holes
Discriminate better around iron.
Slightly deeper for given size.
Don't believe the advertising by a major manufactory of DD coils that show the detecting zone as a coke. It is false and widely reproduced on the internet without checking.
Some coke filled fields the DD is better. Suspect water logged ground also but never checked
The differences are very small. For example coin below two nails sweep along the nails and the DD wins. Sweep across the nails and the concentric wins.