If for some reason you won't, IEnumerable is preferable because it's the interface for all synchronous collections in .NET, including arrays.
@FuncOfT: It would be preferable for regular APIs, but I don't think it really matters much in a WebApi. You don't know what's on the other end of the wire and have no control over what local data type they use in any event, so API design considerations here matter little. There's no difference in the resultant wire transfer, performance or functionality. The only reason to prefer IEnumerable<T> to an array would be if your underlying data wasn't an array to begin with... which we can't determine from the question.
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