I prefer metal.
Metal is an attractive feature to me for it's strength, looks and unique sound qualities, there are brass, copper and aluminium IEM's where the material is intentionally used to achieve the desired sound signature, in this case I don't know if the magnesium alloy affects the sound signature or if Sony only chose it for it's light weight and strength, though I am curious to find out. =)
Plastic to me is a shortcut, cheaper production cost and easier to implement, naturally plastic does not carry any favourable sound qualities like brass or wood, think voovoozela versus saxophone.
As an example, even though the Yuin OK1 is highly praised for it's sound, since it's plastic I feel Yuin haven't put in enough effort, surely plastic is not the ideal material for the sound signature they were looking for, and it's weak and flimsy and unattractive, so I will not shell out $220 for something like that. If they were bronze? Hell yeah.
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As for the FR on the EX1000.
Clarity, definition and edge are in the same frequency realm as sibilance and harshness. I'm sure Sony had the former in mind and the spikes are intentional.
Perhaps they couldn't get the formula quite right in their EX700 and this is an extension of that sonic project, I have not heard the EX700 or this one, but I do favour definition and edge so I am quite interested in these IEM's, I'm sure they'll sound nice with violins.
For those scared of the FR graphs, perhaps the EX800 is more to your liking, which I'm assuming has a dead-flat FR like their studio headphones. If anyone does pick up a pair of the EX800 at some point, please let me know if it sounds like a dynamic-driver, acoustic sounding CK10, because that is really what I'm after at the moment. =)