Plastic.
Bladders to be honest.
Saw a lightweight hiking set up that I am trying to rip off.
Basically, you carry three bladders, and a filter, so you just refill your day one, and use the other around camp. That way you only ever have to carry 3 liters max. Very tempted to do this with one 3 litre bladder, a two litre bladder, and a 2 litre pop bottle.
The main thing that puts me off the metal idea is weight. The bloke at wilderness outfitters has done a nice metal bottle review, sort of a Stainless nalgene bottle, that you can remove all the bits from and cook with.
Liquid fuel I carry in metal, because that is just asking for trouble.