Personally I would take I-90, assuming you're not in a rush and can explore the scenery. Especially if you're doing I-70 back, I think it's more interesting to stay on the Northern route going out.
From Kansas City (Kansas)
, head straight up to I-90, then slow down and take some time through the Badlands National Park
and Black Hills National Forest
. Set aside more time around Bighorn National Forest
and decide if you want to go into Yellowstone National Park
before you get to Portland
I'm assuming you're into national parks and you have time to explore. May won't be super warm yet, so if you're concerned about weather or you don't have much time to stop, then just take I-80.