Probably the road bike, but that depends carry much where you're going.
To the best of my knowledge, bikes aren't allowed on any trail in any national park in the PNW. Bikes aren't allowed on most trails outside the parks either, at least in my state.
They're are a lot of fantastic scenic roads to ride. A lot of them are paved, many more are gravel. Personally I find a MTB on gravel to be kind of boring.
I live here and have a gravel road bike, no MTB. That would be different if I lived E of the crest.