ALL are excellent. I am 6' 2" so my SE shaft is extended pretty much full tilt. I have to do that in order to be able to keep my arm straight...no bend at the elbow...pretty much make my arm an extension of the machine. I would agree 100% with keeping your arm straight..no matter how tall or short we are whichever the case may be...which relieves stress on bicep/shoulders.Wherever the coil falls at ground level then so be it...but definately as little bend in the elbow as possible will definately cut down on shoulder/arm strain and allow for more hunt time.
I also like NHBob's response which gives as much wide sweep hunt time as we can since we all know the slower we go...the more we find with the Explorers. Then once we tire...shorten up the shaft and bring the coil closer to the feet.
I'm tall enough that I have the shaft maxed and it seems to fit me just right...no bend in the elbow...the machine an extension of my arm....the coil not to far away for discomfort yet not to close to my feet for a pretty decent ground coverage.