A former teacher gave me this explanation: "Unu el" estas multe pli ofta en la Tekstaro. Kaj la plej ofta uzado de "unu de" estas en la formoj "unu de alia" aŭ "unu de la alia" ("La muro disigis ilin unu de alia", ktp). In English: "Unu el" is much more common in use. And the most frequent use of "unu de" is in the form of "unu de alia" (one from another) or "unu de la alia" (one from the other). For example: "La muro disigis ilin unu de la alia" (The wall divided them one from the other).
I would add this: "El" is translated as "of", "out of", or "made of" but "de" is translated as "of", "from", "by", or "since". They both have "of" in common, which might be why it can be confusing to know which of the two is more appropriate to use. What if the sentence above were this instead: "One out of every three diplomats is deaf". Would it be easier to decide between "unu el" or "unu de"? To me, it's clearly "Unu el ĉiuj tri diplomatoj estas surda".
Maybe someone else can expand on this. My books are all packed, otherwise I'd try to find a better example.
Estas plia diskutado pri tio ĉe la jena ligilo: https://www.facebook.com/groups/duolingo.esperanto.learners/permalink/481879065307449/
You would typically have one ambassador but multiple diplomats per country. The Ambassador is the "Chief Diplomat", if you will. So all ambassadors are diplomats, but not all diplomats are ambassadors.