I recently performed a number of efficiency tests comparing Logic Pro 9 and Studio One – both latest versions.
In every single test, Logic was more efficient (checking with Activity Monitor) and usually by quite a margin. For example, in one test (using Kontakts and VE Pro) Logic would average about 125% CPU, S1 would average over 200%.
What was really odd was the idle time CPU usage. When everything had stopped playing, and reverb tails were finished, Logic would use about 5-10% CPU. S1 would still be using about 150% - not good for laptops, those fans will never stop running.
Also odd was that S1 would cause VE Pro to use more CPU than Logic as well (160% vs 80%).
So whatever magic dust the developers used in Logic, it works extremely efficiently!