Been thinking about this in the back of my mind awhile, Ni, go! But I'm going to go out on a limb and say Fi and Fe are actually, the same thing. Just that the two are affected with varying degrees of multiple factors. Basically, I think of Fi and Fe on a scale and anything in between is a different degree to which these multiple elements come into play.
Too busy atm to go into depth on this but hopefully I'll be able to post more about this in the psych thread before too long.