Both terms are have been seen quite a bit, but I'd say in recent years whole milk has become the more common, solely due to the preferred choice of milk producers who think diet conscious consumers may be put off by the inclusion of the term, full-fat as it may remind the consumer of the fat they're drinking. Whole milk, although having the exact same meaning, sounds the more healthy option suggesting it is whole, complete and healthy, free from being tampered with - even though it contains the same full-fat milk.
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