Have been using 1/8 inch bungee but just bought one of the manta necklaces to try. As to length my ideal is long enough to go over the head comfortably and set where just ducking my chin will allow me to pick it up hands free.
I found my reg pulled out a bit too easy. Also, the rubber on the Manta is a bit too "grippy" and pulls on wet/dry suit, hoods, hoses, gear, etc. Shock cord slides nicely over whatever it's resting on.
Your mileage may vary...
:cool2: