I'd probably give a slight edge to going eastbound during the summer months. If you plan to travel at any other time of the year, then I think that it becomes more of a toss up.

During the summer months you can still catch some decent scenery out of either Portland or Seattle as you climb up into the mountains. During the rest of the year, much of this is encountered in the dark if heading eastbound, you can only see it by riding westbound.

Glacier can be seen in either direction pretty much year round, unless the westbound train is late by a couple of hours. That does happen, but not all that often.