flash output: profoto d4 vs elinchrom 404

[tmx3]

I was wondering how powerful the D4 is (rated at 4800J) having used the 404 (rated at 4000j) quiet a bit. I understand you cannot compare packs directly on their power as you take into account relfector etc and if i remember correctly, 'efficiency'. Can someone explain this concept to me?? How much of a variation could this be? Has anyone used both?