My bike selection is determined by the surfaces I will be riding on, not my riding position, or the distance of the ride. If my route is all pavement I will be on my road bike (race geometry, slammed stem). If it is a mixed-surface ride, my pick is my gravel bike. If it's all (or almost all) dirt, with significant rough terrain, I will ride my MTB. That said, I sometimes do the same rides on both my gravel bike and MTB.
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"Swedish fish. They're protein shaped." - livedarklions