Agree with your post Function.
In some cases they essentially do very similar things. For example, - Atropine is a muscarinic (parasympathetic) ANTAGONIST - i.e. a parasympatholytic - Adrenaline/Epinephrine is an agonist at all adrenergic receptors i.e. a sympathomimetic EITHER can be used in the treatment of bradycardia (slow heart) depending on the circumstances. The parasympatholytic will reduce the parasympathetic effect slowing down the heart, and the adrenaline will increase the sympathetic action speeding up the heart. A lot of the time however, it will probably depend on the specificity of the drug as to which one is used. I.e. which receptors it works on in order to bring about a specific effect. There are cases where the drugs are not interchangable and this is because of differences in sympathetic/parasympathetic supply. I can't think of any examples at this minute.