From a business viewpoint it must be a combination of cost of purchase, cost of maintenance (inc cleaning when required) and how long it will last for. From the environment viewpoint, carbon costs need to be added. Steel is just as likely to have been recycled as plastic.

Additionally customer preferences need to be considered. I am concerned about how freely water will drain from a plastic trolley base when it has been outside in a supermarket car park on a day like today (vet wet for those who read this in the future).
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Simonridout