I would pick the Cascades. The Talgo equipment is sleek and nice, and the on-board bistro cafe has a much better selection than the cafe on the Coast Starlight (it doesn't have a diner, but who's going to eat in the diner on a short trip like that?). Even if you don't get the Talgo equipment as mentioned above, the Cascades will be more reliable--the CS often gets hours delayed en-route from Los Angeles, whereas the Cascades departure times are much less variable (not perfect, but on the whole better than the long distance trains).

That said, I think that if the CS is selling tickets at low bucket, it ends up being a little less than the Cascades' standard fare, so if the CS is cheaper and you're OK with a potential delay getting to SEA, no reason not to take it. Same track, same scenery, and the other factors aren't huge.