They both occur in Illinois and considers by some to be a variety of the same species. according to the article I posted above they can usually be seperated by the pubescence of the leaf underside.
"Comments: Smooth Arrow-Wood is very similar to Viburnum dentatum (Arrow-Wood) and it is sometimes considered a variety of the latter species. Smooth Arrow-Wood has leaf undersides that are hairless (or nearly hairless), while the leaf undersides of Arrow-Wood have fine downy hairs."
