Exchange Colo would probably be better. If you look at it from a capacity point of view, the Bandwidth is really in the SF Bay Area. With Seattle, most of the carrier Sonet runs at lower speed then in the SF Bay Area.
the westin business center routes the majority of telecommunications in the northwest i believe, which puts them as a very rich peering point.
companies like sextracker (fci) host there for instance, and have a multi-gigabit network internal to the building. at least, that was the way it was 3-4 years ago, i can imagine it's only larger.
SF bay area would have the advantage of being close to PAIX... it's probably pretty fair. especially if you're only getting a dedicated server or two, you won't notice much difference. if you were going to be buying multiple FE's or some gigE, it could make some difference then.