I went with a 14/1 and slowed it down to 600 RPM. This provides exactly 1 ½ times the power of the 6/1 and ¾ the power of a 12/2 when they are operating at 650 RPM.
There is only one injection pump which lowers the loading pressures on the idling gear.
The extra heavy flywheels also help when the engine speed slows to this level and provides an acceptable level of stored kinetic energy to maximize engine life and smooth power output.
It is direct injected for improved fuel economy as well, some people like the indirect injected. I have both and they are both great, but the direct injected offers a slight improvement in fuel economy if you are running only diesel/home heating oil.
I like the single for the durability.
Diesel Guy