Hewitt definitely has the edge. Probably one of the top volleyers of tour out of players that predominately play singles. He didn't rely on them that much because he never had enough power in his groundstrokes/serve to get to the net consistently.
Awwww dangit! I think you got me there. Hewitt is a way better doubles player than Nalby. Which probably means he's a better volleyer. Most people forget that Hewitt won the US Open in dubs