I was using gnome-shell and I forgot I wasn't using Compiz. I didn't pay all that much attention to my battery life as I think I was plugged in. Has anyone taken notice of the resource consumption of the two rendering methods?
I havent found anyone who has done a direct Mutter vs Compiz resource usage study.
Its arguable that Phoronix has done the closest to this looking recently at performance comparison between gnome-shell (Mutter), unity (compiz) & KWin using both NVIDIA and ATI graphics.
My reading of the results - and making the assumption that better performance is equivalent to less resource usage for the same work - is that gnome-shell and unity were perhaps equally resource hungry when using NVIDIA drivers, whereas gnome-shell using ATI was more resource hungry than Unity.
However, KWin was better than both gnome-shell and unity.
Usual caveats should apply when trying to interpret Phoronix results & methodologies - arguments should be directed to the Phoronix forums!