Great subject !
I am sure there is confusion talking about sustain as to what it really is we are talking about. If you strike a string and let it ring; it is normal to have sustain and that is the one that comes from the string. For the instrument, it is the sound that lingers after you have stopped the string for ringing. That is my uderstanding of it in this case. Too much sustain would make the sound muddy and, not enough would make the sound dry and sharp.
To me, it all depends of how you use the instrument. I would prefer more sustain on a low G (more finger picking) and less on a high G (more strumming)