x86 systems (and CISC in general) do more in every clock cycle than RISC. However, ARM is a clean, simpler architecture, less encumbered by historical considerations than x86.I don't want to start such a war, but the fact that CISC systems are more faster in therms of calculations follows logic..... The only reason why RISC is used is the licence, welcome in the world of economics.... A few month a go I red that the AMD/Intel x86 systems are more power efficient because they are calculating much faster as there ARM counterparts, so they can compensate the bigger power drain....
In addition, the focus with x86 has, until recently, been raw computing power - leading to such horrors as the pentium D.
ARM has seen far more use in low-power, portable and embedded devices, and is designed with those in mind.
It takes less energy to perform an operation on an ARM processor than an x86, but you have to do more of them for any given calculation.
Since a computer does little mechanical or chemical work, efficiency doesn't really make sense. What you can talk about though is the energy required to perform a given calculation.
A good SOC will have a low energy:computations ratio across a whole range of CPU loads.