After doing research I'd say the SCAR series would probably be generally more efficient due to the fact that:
It has a light and heavy variant (light variant chambered in 5.56x45, heavy variant chambered in 7.62x51[.308])
It has different models with different barrel lengths depending on the situation
The AR was very close though, because although specific models only have 1 barrel length and most ARs chamber 5.56x45, it was specifically designed to be lightweight.
Really depends on the situation I suppose