It's a good comparison. Both were large centers (and although neither was soft, neither played with much physicality). Their careers largely overlapped. Both players were clearly the best scorer on their team for the majority of their careers. Both were very consistent. Finally, both were very loyal to their teams (and both had disappointing stints elsewhere in their last season).
The arguments for Sundin are:
Although Modano was consistent from year to year, and had a long prime, Sundin fared better in both of these areas. The Swedish centre was top 40 in scoring every year from 1992 to 2008, while the American was top 40 in scoring most years from 1992 to 2006 - except for 1995, 1998 (just barely missed) and 2004. (But if you raise the standard to finishing in the top twenty, Sundin did so seven times in twelve years; Modano did that five years straight, and seven times in ten years - so it's very close).
Sundin was better on the international stage. (Modano was very good, but there's no argument that he was better than #13).
The arguments for Modano are:
Modano had a higher peak. (Don't tell me that Sundin had the better peak due to 1993; scoring was really high that year and Sundin was only 11th in scoring). Sundin never placed in the top ten in Hart voting. Modano did so three times (1997, 2000, and 2003). Modano earned roughly twice as many Hart votes as Sundin over the course of their careers. Modano also has more years as top ten scorer (3-2). (Counter-argument: Sundin has two years as a year-end all-star, as opposed to Modano's one).
Modano was a much better defensive player. Sundin was fairly average. Modano was excellent from 1997 onwards (he peaked at 3rd in Selke voting, and had four years in the top ten).
Modano was a better playoff performer. It`s true that Sundin scored a bit more on a per game basis. But Modano maintained his high average during deeper playoff runs (five conference finals appearances vs two), and he played more games while past his prime (37 games played after age 32, compared to just eight for Sundin). Modano leading the playoffs in assists two years in a row (and finishing runner-up in scoring both times), while being a top two-way forward, is much more impressive than anything Sundin accomplished in the postseason.
Overall I'd say this is close, but still clearly Modano. Their offensive output is very similar. Sundin was better internationally, and his prime was a bit longer and more consistent. But Modano got more Hart consideration, was better in the playoffs, and better defensively - he was the better player and had the better career.