Well, as a veteran of both the Slog and the drive from Bakersfield to Sacramento (or for a three hour equivalent, Fresno to Sac, which I will call the Fog) I would vote for the latter as much, much longer. The drive from Portland to Seattle has turns. It has trees to view. It has rest areas. It has paved roadway. It rarely has fog. It's no contest.
Although it's somewhat out-dated. The last several times that I've been forced to make the trip (confirmed by 3 trips by my niece) have taken 5-6 hours!!! It is truly the road thru hell.
Veteran Slogger as well. Worked in Seattle weekdays, lived in Portland Weekends for a lotta years, relatives spread out between the two cities, friends and gigs in Longblink... Oy!
I used to joke that you could take a picture of 1 square inch of I-5 pavement between Portland and Seattle, and I could tell you the closest mile marker.. north AND south.
Being a Texan and not knowing much about the Great Northwest we got on the Slog out of ignorance .Last year after The Oregon Gathering we were looking for a direct route to our sons home in LaConner Wa . OOOPS!!! Why didn`t some one tell me?
Well, not doing it as a commuter, I schedule the time I hit the road carefully. At the right time of day it is 3 hrs from Portland to the Mukilteo ferry north of Seattle. The trick is timing it to avoid traffic in Portland, Vancouver, Olympia, Tacoma, and Seattle, and there aren't many ways to do that. Wednesday morning I'll probably leave Whidbey about 9 and go through Seattle about 10, Portland 1-2, and that works pretty well. Obviously would not work for a commuter, though.