Should I play the violin or viola?

Both are great instruments and require as much dedication and practice as any other string instrument. In my personal opinion, the violin is a much more versatile instrument with a much greater repertoire than the viola, not to say that the latter isn't a fine instrument. I would start on violin and refine your violin skills. Then, if you're still interested, switch over to viola. The two instruments are actually fairly similar, the only difference being that the viola is tuned a fifth down, meaning that it has a lower sound. Also, the intervals on the fingerboard are slightly larger, but not enough to really affect your playing substantially. Good luck though.