It's highly possible, maybe even probable, but ultimately Serena is the better player because she can overwhelm the opponent with her game and is mentally tough throughout her matches, something that still cannot be said of Justine. Because Henin prefers to use her opponent's pace rather than generate her own, she still struggles a lot against the slice-and-dice, topspin clay merchants who try junking her around and play heavy, deep, looping balls to the baseline. Mind you, she is the one who thrashed Martinez 1 and 0 at Wimbledon in 2001, so she knows how to handle some of those sorts of players.
And I would argue that while Davenport is impeded by her mobility against Serena, it isn't Capriati's lack of speed that lets her down, it's her serve and obvious mental frailties, which were vividly exposed in the two crucial matches they played on clay last year, at Rome and the French Open, where she blew big leads and lost the match.
Say it Quietly, I'm White but I'm Really Rather Contrite