I learned guitar first, then I learned bass. I did that because I wanted to play guitar (and still do, primarily). Wanting to learn bass came later. Learning guitar for the fretboard, chord structure and general theory helped me pick up bass quicker. Knowing the layout of a keyboard from high school and college music classes helps me visualize scales. But as everyone says, learn what you want to play. Then learn something else you want to play. It's great to build on previous experience with an instrument for another instrument.