Personally I like the feel of 6mm perlon for cordelettes. It ties well, it's strong enough, and it's dirt cheap.
Never seen an EDK flip, but I like some overhands on the tails of the knot, as weighting and unweighting seem to shorten the tails. I think it's important to have long tails on any EDK.
Gary, yes it is rare to untie a cordelette for rescue or hauling purposes, but it's nice to be able to do so easily when you want to. The fisherman's after it has been weighted after a season of climbing can be a pain to undo in the cold, etc. I don't think it matters that much, but the AMGA says to tie 'em that way so there's that, if you are into the guidy tricks.