I think you can plan on smaller margins with the FES since the engine-not-running drag is much lower with the FES than a stuck-out pylon. You'll have to make your own decisions of course.
That's what I find so cool about FES. The JS3 RES has a pylon with motor in front, whereas the GP15 Jeta has the motor in the back, which at least allows to fold the prop and minimize drag in case it gets stuck. I'm not sure if the Antares prop folds if the pylon is stuck?

FES is a great illustration of KISS: keep it simple, then fewer things can go wrong - and the 1 point L/D penalty is OK by me. I'm on the market for a FES glider and currently debating between LAK 17 and Shark 304.