From the Seattle Summer Events Guide:

It’s been eight years since Manchester United—easily England’s most popular pro football club—has ventured to Seattle, and Sounders-fueled soccer-mania had yet to grip the city back in 2003. (Granted 67,000 fans showed up to the match at Qwest Field, where Man U played Celtic United, but still.) So for those who need to brush up on the Seattle side’s international opponent in today’s friendly, let’s see how the two clubs stack up:

Payroll
Man U: North of $200 million
Sounders: $3.06 million

Championships
Man U: 19, including the 2010-11 English Premier League championship
Sounders: 0 MLS championships, but two U.S. Open Cups

Average Attendance
Man U: 76,000
Sounders: 36,000 (although more than 66,000 are expected for Wednesday’s match)

Fans
Man U: The Red Devils’ fans have a history of being, well, devilish. In 2007 and 2009 police arrested more Man U supporters than those of any other club in the EPL. In February 2011, four fans were arrested for lighting flares inside a French stadium. One month later, three fans were arrested and six ejected from a match for, among other things, taunting their Liverpool opponents with chants about two stadium riots that killed nearly 140 Britons in the ’80s. And that was at a youth match.
Sounders: No arrests, unless you count the rumor that a Seattle supporter got nicked in 2009 for accosting a Portland fan with a Sounders flag.

Seattle Sounders FC plays Manchester United July 20 at 7 at CenturyLink Field (same place, new name).