If you want cheap and cheerful then go for plastic, if you crave the lightest bike (or just want to show off a bit) then go carbon, and if you want classic styling or care about durability then pick from the metal options.
Plastic bottle cages can rival the low weight of carbon, but come in at a much lower price on the whole. Don't be fooled into thinking they're flimsy though, as many are reinforced with glass fibers to add strength.
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