Is returning IList worse than returning T[] or List?

Maybe this is not directly answering your question, but in .NET 4.5+, I prefer to follow these rules when designing public or protected APIs:

• do return IEnumerable<T>, if only enumeration is available;
• do return IReadOnlyCollection<T> if both enumeration and items count are available;
• do return IReadOnlyList<T>, if enumeration, items count and indexed access are available;
• do return ICollection<T> if enumeration, items count and modification are available;
• do return IList<T>, if enumeration, items count, indexed access and modification are available.

Last two options assume, that method must not return array as IList<T> implementation.