There's absolutely no comparison between Modano and Demitra in terms of defensive play.
The evidence (aside from watching them play):
- Demitra got very little ice time on the penalty kill until after the lockout (when he was no longer a top scorer); Modano was one of the top penalty killing forwards in the NHL (from 1998 to 2004, he was 18th overall - with most of the names ahead of him being nowhere close to him offensively)
- Demitra got votes for the Selke trophy once in his career (he had high plus/minus - this was the year his teammate Pronger won the Hart). Modano got at least one vote for the Selke trophy 10 times, with four years ranking 6th or higher
- Plus/minus has limited value but it's worth noting that Modano led his team in 1997, 1998, 1999 (tied), and he was 2nd in 1995, 2001, 2002, and 2007. Demitra had two years where he was 1st on his team (2001 and 2006) and one year as 2nd (2000). Plus/minus has limited value, but this is a really one-sided comparison. (Especially since, in one of the years Demitra led his team, he was clearly no longer in his prime offensively).
If you compare Demitra and Modano strictly in terms of regular season offense, in terms of best 1-2 seasons, it's close. But Modano was a vastly better two-way player, and a much stronger playoff performer. Modano pretty clearly had the better peak unless literally all we're doing is comparing the number of points they scored.