I've tested coils on the AT Pro for Kellyco just recently and the results told me that the Garrett 5x8 or the NEL 5.5x9.5 Sharpshooter would fit the bill perfectly. Both are excellent in terms of sensitivity, separation and relative depth for coils of their size. With both coils, I was able to detect a good target with an iron nail even on top of the good target. The tone changed when I place the nail on the target but you'll be able to learn those distinctive sounds after one run through a test scenario.
Just the view from my foxhole...