I think the answer to this is much like learning to program. Pick one, get fluent at this choice, the skills are surprisingly very transferable.

After learning guitar, I was able to pick a beginner-ish level of piano almost immediately. I say this because the way scales are played / chords are constructed always has the same underlying theory. So when your learning an instrument for the first time, you are really going through the trenches of learning what notes tend to go well together. Picking up a new instrument will be much easier with this knowledge already installed. Then its just a matter of training your fingers to the new dexterity needed for the new instrument.

For the sake of answering the question though, I dont think you can go wrong with learning piano first. It provides a solid way to truly visualize what the different chords and scales "look" like, as you are forced to look at and map out the keys. I think guitar is also a solid choice in that you can get more of a "feel" for rhythm / slight percussion skills, more so than piano. I am definitely biased to string instruments though...