Mountains are subjected to very intense erosion that breaks the exposed rock beds and pieces down physically and chemically. The products are moved down slope due to gravity slides, running water, etc. and are eventually deposited in the adjacent river/stream valley where they undergo further chemical and mechanical weathering. The residency time on the mountain top versus in the valley is very different. The longer the time spent being decomposed the more dissolved compounds are released into the forming soil. So, valley soil will always be richer than mountain soil.
I will say that a Valley has more fertile soil, because there are less erosion factors (water, air, change of temperatures, pressure compare with Mountains, most of the Valleys are crossed by rivers that supply enough water to make the plants grow.