I used to have the double fisherman knot, until Bob Sherwood ripped it off (it took quite a pull) to 'prove' how easily it comes apart... Ever since I've just had the shock cord through the zip tie; it's adjustable which is good I guess. Bob is on the 'GUE Training Council.'
In any case I think they both work fine, one is adjustable and the other is easily removed/replaced.
Funny how different instructors teach different methods... even different training counsel instructors.
There were a couple of thread of DecoStop where the proponents of each method really went at each other. Both sides had good points, both sides had upsides, both sides had downsides. In the end, you have to decide for yourself which method you feel safest using. For me, the ability to have it release if I needed it to seemed more important than the very small risk of having it release when I don't want it to. You can plan to overcome one problem, you can plan for two, but when the third failure comes along, sometimes your ticket is just up. That said, I have yet to read of a single dive accident report attributable to a necklaced backup failing in either way. They both work.