I'll bite:
It just depends on the PIAS staff and location. The one in Seattle has more Discraft, and it looks like they order from Dynamic because they have Misprint Cryztal and whatnot that I've only seen there. They also have the punch card system computerized and a small used selection.
Woodinville picks up whatever's hot but never restocks it. They had a few MVP and a decent stock of Latitude last year (which started out marked up about 20%), but it's down to one Latitude and no MVP (and has been for six months). Now they have a massive supply of Prodigy protos and even a few Dynamic Discs. Never seen more than three used discs for sale though, and those are usually beat up DX for $5. Decent X-Out selection. This was the first place I bought discs, and when I made my selection the sales dude (who plays) offered no input than "Gazelle. Nice disc. Never thrown a Monster though." In other news, I have a Monster in my stack of "For Sale" discs that will be popping up on the MP soon.
Lynnwood has the most X-Out discs and a few more used discs than Woodinville. They only carry Innova and Discraft though, and sounded surprised when I told them that Woodinville had Latitude and that Lat is hot right now.
So of the three, Woodinville has the most discs and the most variety, Lynnwood is the best to stock up on cheap throwers, and Seattle is your best shot at finding a decent used disc or a disc that you won't find anywhere else in the area... I'll just keep buying from Lowell Shields or the MP though.