ViolinI'd recommend the piano because you'll inherently learn more about music theory and how notes work together, and that will benefit you in many ways. Definitely Keyboards provide a very clear, visual analogy of the notes they create and this facilitates understanding the more abstract ideas in music (it can distract from it, too, but it helps more than it harms).
Also, a piano can simply cover more possibilities than a violin -- not that a violin doesn't have a very important spectrum for emotional expression and a well-deserved reputation as an instrument of technical and expressive mastery.
If you become familiar with music on a piano, you can then move on to violin and whatever else you like with that very useful foundation to support you.
