We stayed at POR in 2006 and were in AB25. We used the West depot, and for our entire trip (at least when we were on the bus) the West depot was the first pick-up, followed by North, East, and South. The South, or Main, depot was last every time - and it was also the most crowded. Since the West depot was first, we of course always had a seat. HOWEVER, alot of people report that the Main depot was first during their stay. The only thing I can think of is that the order of pick-up changes from time to time, maybe based on the time of day. Anyway, I think the West depot is great, because it will always be either the first or second stop, and it is not crowded like the Main depot.
Hope this helps!