so on a fuel/liter of water melted the whisperlight is more efficient, just slower?
Don't know. The XGK always cooked faster for me so I would guess that less fuel is used in the process, making it a better choice. If you put two stoves, a wisperlite and an XGK side by side, the XGK would run out sooner. But since so much more heat is released, the cooking time is much shorter too.