With I84 you avoid two very large passes. On I82 there is Umtanum pass between Yakima and Ellensburg and on I90 you have to go over Snoqualmie. The posted freeway speeds going over Snoqualmie always gives me the willies even in a small car. There is also the optional Tiger Pass to bypass Seattle traffic, but it is 2 lanes for a lot of the way and always seems to have traffic on it.
I-84 follows the Columbia River and is as very flat in comparison, and I5 from Portland to the Gig Harbor exit is also flat. You also get to travel through the town of Hood River. Note: Why isn't there a Rpod Rally in Hood River, it seems the obvious choice...
While the Washington side of the Columbia Gorge (SR-14), is two-lanes, parallels I84, and a bit more hilly, I would recommend it as it's a beautiful drive and you can see the ("where in the Sam Hill") Mary Hill Museum and the Stonehenge as well.
Sean