if I was riding with 9/10/11/12 rear cogs, I might opt for a double chainring as it would give a broader range of gears.
But why not go triple at the front?
From my experience, 3*9 never worked at the extremes of gear combinations and was too temperamental.
It seemed like one step forward, two steps back.
So I went ‘back’ to 3*8 as it suited my riding style (wide range of gears, extra low gears since I ride 29′, and the reliability to work regardless of the gear combinations.)
A single chainring would probly be best for pedelecs as you’re letting the motor do the bulk of the legwork.