Should I Learn Piano or Acoustic Guitar?

With musical instruments there’s always a natural talent that all people have to choose one instrument or the other. I would suggest to try them both (at a store maybe) and see which one of the two you feel more comfortable with.

Some pointers:

1. For the piano, its better to have long skinny fingers. For the guitar, skinny fingers are useful but not fundamental.
2. If you care about how "femenine" your hands are, choose the piano. The guitar strings make calluses on the tips of your fingers. At the beggining it doesnt look too pretty, but with time the tips of your fingers will get a coat of "hard skin". And also, your fingernails. You will have to keep them nice and short.
3. Price. If price matters, go for the guitar. The most expensive (entry level) acoustic guitar will hardly match the price of a half decent piano.
4. Popular applications: You see, most popular music is composed with either a piano or a guitar. If the songs you like are more guitar-performed, then choose the guitar as you will enjoy it more.
5. Portability/Posture: Clearly, the guitar wins in this department. You can play it sitting in a chair, on the ground, standing (with a strap), laying on your bed. This, for me, gives it an emotional value, as you can embrace your instrument. The piano is much more impersonal.
6. Coordination. This is a matter of practice, but just take the following into consideration: On the guitar you´ll move both hands in different ways. One for picking and strumming and the other one for chord formation. On the piano, both of your hand will play the keys, AND your feet will be in control of the pedals. So, in a matter of coordination, the guitar is a little bit more simple.
7. Playing the music: Every note on the piano is a single key. Every note on the guitar is a single string, sustained at an specific fret. The piano is easiest here, because the notes are already laid there for you to play in a single movement. On the guitar, by contrast, you have to "make" the note in two movements, one with every hand.
8. The notes. In short, harmony is simpler on the piano. This is the way the chords are played through the instrument. The piano wins here too. The keys are the notes, you cant miss them. You can see the harmony spread out before you. Its glorious. You see the notes directly, so the chord formations are pretty much the same on different tonalities. On the guitar, because of the way its tuned (standard tuning) the chord formations are played in different ways. So, for instance, a major chord will have various hand positions on the guitar, and just one on the piano.

Overall, the piano is a more "femenine" instrument. I know more female pianists than guitarrists.
One thing to have in mind is, that once you learn one instrument, you can take on a second one and it´ll be easier! so dont be afraid to choose one or the other. If you want you can learn to play both!

Good luck!