Gotta make that a priority, both are worth seeing! There is lots of cool stuff in between too!
Echoing what
@footballer said, so much to see out west. I'm a bit partial to Montana, Colorado is amazing and I love that they promote off-roading they way the do with their public lands. Numerous 12,000'+ peaks to drive up. It's stunning!
Utah is the same way with off-roading on their public lands. All the parks that @football mentioned in Utah and don't forget Moab, which is absolutely gorgeous.
If you visit Yellowstone, then I'd suggest you fit in Glacier as well.
Hopefully one of these days my wife and I get to tour the US for a year in a skoolie. #DreamTrip I want to spend a month in Alaska.
The San Juans in Southern Colorado, small towns like Silverton, Ouray, Telluride surrounded by 12k mountains.
Picture below is from 2017.