Are all emulsifying agents created equal
From a chemistry standpoint, yes, there is a difference. That depends on the desired effect and composition of your emulsion, so there are more appropriate emulsifiers for water-on-oil than oil-on-water, and also depends on what size of micelles you want forming, texture and etc. 

A complete explanation is a whole course in university and depends on a lot more details, so that's the condensed general version.