The deciding factor is portability. You can pick up your acoustic guitar and go anywhere and be life of the party. With a piano, you really are limited to having one at your intended destination. And then hoping it has been tuned properly. If you want to be a full-time musician, it doesn't matter so much, because you will always have good quality instrument to use wherever you perform. Look into your heart and see which instrument talks to you. ...and the musical genre in which you want to specialize!!!
Guitar! There are so many guitar legends and icons it's such an amazing instrument! So many extremely talented guitarist to name a few: Jimi Hendrix, Keith Richards , Slash, Angus Young, Eric Clapton, Chuck Berry, Jimmy Page, and Joe Perry.