I have traveled in both directions on the EB and Eastbound offers a better chance that you will get the best views regardless of if the train is on time or not. As a previous poster mentioned, a disadvantage to the Eastbound route is needing to rise very early, particularly if the train is on time. On my just completed EB journey, I awoke at 5:10 A.M. The shower was ready for me, dressed and was ready when the Dining Car opened for Breakfast. However, the train was running late. That gave me time for a leisurely, tasty Breakfast with plenty of time to find a good seat in the SSL for our journey through the Park.
I did the Portland segment of the EB once, but I prefer going though the Cascades. Just wish there was enough equipment to have a Dining Car and a SSL on the segment from Seattle.