I'd probably grab an "A" seat. I used to fly into SEA often, mostly from DEN, and that's the side where I seemed to get a Mt. Rainier view the most often. Plus, if the plane flew out north of SEA to turn around for the approach, it seemed like we'd fly up over Bellevue/Redmond/etc., and then turn around and approach over Puget Sound...throughout that entire process, the "A" window has more of a view.
Of course it's a crapshoot and I have no idea of we were on a different pattern coming in from Denver than you would be from PHX. One thing I'm pretty sure of: I don't recall ever being routed way out west of Seattle for the approach. It was either a straight shot in from the south or we'd fly up north over the eastern part of town to turn around.
Sorry for my rather crude descriptions
...enough pilots read these boards that someone can probably provide much better details.