If you read the text of the spells:
Amplifies magic used against the targeted party member, increasing damage taken from spells by up to 15 and healing spells by up to 30. Lasts 10 min.
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Dampens magic used against the targeted party member, decreasing damage taken from spells by up to 20 and healing spells by up to 40. Lasts 10 min.
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You can see that Amplify Magic should be used when a party member (or yourself) has someone healing you, and they are only taking melee damage. As a priest and a druid, the +healing increase isn't worth the extra spell damage except in special cases (one mob, 10 healers) If you amplify your tank against a melee only mob, then the extra healing may help out. This relies on you knowing what is spell damage, and what is melee damage =)
Dampen Magic helps you against casters, or a party member against casters as the above posters have stated.