Both the AT Pro/AT Gold will work great for coins and most relics, however, for hunting gold nuggests or tiny tiny objects, the AT Gold is the better choice because of it's true all metal mode + threshold adjustments. The at Pro might do a bit better for coins and relics because the pro can be adjusted by adding or removing notches without depth loss, whereas the AT Gold has pre-programed the unit for gold searching, relics/coins or just coins, without being able to remove any notched. The AT Pro will ground balance to salt water, the AT Gold cannot be ground balanced to salt. And of course both are water proof.
In your case, go for the AT Pro