I would go with Roddick, based on this past year. Andy has finally figured out how to make good use of his weapons. But I never underestimate Hewitt --- I don't think we've seen the end of his tough fighting style!
On a related note, too bad we haven't seen them play each other since 2001!
All three of their matches were that year, and all were won by Hewitt. But Roddick has improved greatly since then, so who knows how they would match up today! In 2003 Roddick was able to defeat other quick, consistent players like Schuettler, Coria, Ferrero, and Nalbandian, whereas I think he would have had more trouble in the past.