IMO, Viduka complements Ruud - the former is a true targetman - strong on the ball, great header of the ball ( one of the best in heading duels & headed goals ), holds the ball well, distributes well, has a great shot & difficult to shake off - Ruud on the other hand, is not a targetman, but a great poacher ( altho' he's improved in his holding & distribution skills, he's piss-poor in his heading abilities, still falls over too often & cannot shoot beyond 15 yards - none of his 98 goals came from outside the box! )
Viduka would have opened out more chances for Ruud, pulled defenders away ( Ruud's now marked by at least 2 defenders nowadays ) & would have added goals as well!