This is how I believe it works:
Effiency mode keeps the phones battery at 80% at all times, it doesn't try to always get the phone topped off to 100%. So it's more efficient for the battery Mod, it's not working so hard to try and keep your phone at 100%.
The other mode will always try to get your phone to 100% and drain the battery Mod very quickly.
From what I have always read our phone batteries work best between 20% - 80% anyway, so let the Mod battery do less work and keep the phone at 80%...the most efficiency.