I realize in these economic times a teacher is sometimes prohibitive. However, utubes can only go so far, for both instruments. If faced with a choice, piano, only because the chance of causing irreversible damage to muscles, ligaments and tendons is less. Basically, playing a keyboard, the stressors are less, as both hands are doing basically the same thing. And you're sitting.
One must be aware of the back, shoulders, elbows, fingers and more. How much more when we are talking about an instrument whos playing involves all these with different stressors for each hand and arm doing completely different motions. Just standing and holding the violin under the chin takes practice.
And if one does not have a professional instructor noticing any motions detrimental to the body, I cringe at the damage that can occur. Yes, the same arguments can be put forth with the piano, but with the violin, other instruments such as flute and trumpet, one can really do harm.
Non withstanding the asceticism of which sound you love so therefore pursue, (ear training with the violin noted), the bonus of playing a piano and accomplishing playing a 2 hand composition quickly is a plus. I am approaching the original question basically from a standpoint of which instrument, is studied without an instructor, has the potential of causing greater harm physiologically.