I think Southcenter makes the most sense too. The area right around the Seattle service center would be confusing for out-of-towners I think, and can sometimes suck with traffic, especially with game days which throw road closures and massive amounts of pedestrians into the mix. Not to mention, there's nothing within walking distance except for the stadiums, well, unless it's a little further north closer to Uwajimaya.
Southcenter has plenty of space, and gglockner said, it's nicely located at the junctions between I-5, 405, and 167. Depending on exactly where they place it, it could/should be close to fast food, the mall, other shopping, bowling, or even perhaps the Family Fun Center.