How do I arrive at this seemingly outlandish conclusion? From the Sounders side, it's simple. With Qwest having a proposed MLS capacity of 24,500, and over 19,000 season tickets sold, it seems pretty likely that the club will sell out every game. If necessary, they might even open up some more seats (the Hawks Nest, to be specific), bumping the capacity to around 27,000. For this exercise, I'll play it safe and give the Sounders an even 25k, not counting friendly matches.
The Mariners, of course, have been trending in the wrong direction as of late. With last season being an abysmal failure by any measure, the attendence dropped to mirror that. For the first time since the magical 1995 season, the team drew less than 30k people - just over 28.5k, to be exact. They dropped by five thousand fans in one season. Now, unsurprisingly, changes happened for the M's over the offseason, including a new general manager who isn't a complete moron. And in the long term the club looks to be on the right track. But they aren't going to win this season, and a lot of fair weather fans have bailed on the club. The casual fan doesn't want to see a team that loses every year. And with the economy being in the toilet, another drop of a few thousand fans per game seems likely, putting the Mariners below the Sounders.
Of course this doesn't mean the support for the teams will be equal. That would be an insane suggestion. The MLB season has more than 5 times as many games, giving the M's a huge advantage in total attendence, and baseball (unlike soccer) draws historically well in Seattle. But for the Sounders to draw more fans per game would be a major victory, albeit largely a symbolic one. It would be a first in MLS history. And yet another sign that the club is primed for success.