Also, don't forget about the 2 sections, to SEA and to PDX. The SEA section has the Cascades, a full diner (1/2 lounge maybe?), and no SSL, while the PDX section has the Columbia River Gorge and the SSL, but no diner (sleepers get a delicious but boxed dinner).
In the winter (my favorite!), going west is the best to catch either of these, as it is in the morning with the daylight. Going east in November/December, sunset is around 4pm, before #8 and #28 even depart. After the March time change, the Columbia R. and Cascades should mostly be visible in daylight (sunset 6:50pm). However, by early October, it is sunset at Bingen (6:30pm), and is dark by the fall time change. In contrast (going west), the latest sunrise of the year (just before the fall time change and around New Year's) is at Wishram (7:40am), and more east if the train is delayed.
Drawback is, going west, Glacier Park will be completely dark through Standard Time (even when it's on-time), and a little lit from the DST change (sunset around East Glacier Park 6:50pm on March 12). Summer is the best for going west (thereby seeing both Glacier and Cascades/Columbia R.), with the train past West Glacier, if on-time, by sunset, and around Wenatchee (7) and Pasco (27) at sunrise.
Going east, it is always sunrise by Whitefish (latest sunrise of the year is 7:40am).
All Sunset times got from timeanddate.com. I am a local of the internet, not a local of the Northwest.
Overall, I would probably recommend going east after the Spring time change. That way, you're likely to see most of the Cascades/Columbia R. and guaranteed to see Glacier Park. There is probably snow there in mid-March as well.
I was myself dreaming about taking the trains around this area (probably not happening though)!