I prefer I90, but we come from the south. It depends on the city traffic on the Westside, but getting through Bellevue etc. is a drag. I always like to get to the Eastside and open country ASAP! I can say I90 is a pretty easy tow and somewhat less windy. Also it's 2 lanes or more all the way so easier to pass/be passed (some trucks will be going really slow!).
2012 Jay Flight 29QBH
2003 Dodge Ram 2500 Cummins H.O. 48RE
Equalizer 4-pt sway control hitch
Prodigy P2 brake controller
2006 Jayfeather 29Y (past Jayco :))