Which is the better string instrument?

Beauty is in the eye of the beholder. Some people really like the sound of one over the other. If that's the case , go with the sound you like best.Listen to some recordings of both instruments and see if one sound grabs you more. Cello is much bigger and harder to take around. It is usually a more expensive instrument. You always sit to play the cello. With violin you can sit or stand but it is always best to stand in the beginning for better posture and posture is important. Holding the violin is more challenging at first, but after a while you get accustomed to it. If you still can't decide and you have the opportunity to rent and try both instruments that might help. As a teacher of violin, I find someone with a good knowledge of music will go much faster in the beginning than someone who has no knowledge of music. Hope this helps. If not, you can always toss a coin.