I played guitar for 20 yrs in local bands, then stopped to raise my kids. A church was starting a contemporary praise band and asked me if I would play bass. I only knew very basic stuff about bass but I decided to give it a try. I studied bass tips of web sites, bought books and videos. A bass player has to keep moving, doing scales and riffs, all the time or it will sound amateurish. I learned TONS of things to do on bass. I was a bass playing fool. Then the guitarist quit with no notice and I had to switch back to guitar. Guess what? After four years of bass, I found that everything I had learned on bass transferred to guitar and has made be a better player. So learn both.