RE: Which motor for Ef-1 pylon?
Doesn't seem to any clear favorite.
The Arrowind and OS are a little lighter, so if the plane you are building needs less nose weight, that might be an option.
If you are putting a Shoestring together, the e-flite will be much easier to install.
The Arrowind mounting is different and the OS is a larger diameter than the e-flite and it is very tight up front in the cowl on the Shoestring.
Cowl to far forward and you rub against back plate. Cowl too far back and the outside of the motor rubs on the inside of the cowl.