Both violin and viola are lovely instruments but producing notes and tone is quite different from the piano where you produce the same notes every time you press the keys (and almost that when you key the flute, surely)? Strings take a lot of work at the beginning but once you get somewhere they are very satisfying to play.
For viola you need a teeny bit longer reach and bigger stretch but I have heard of small instruments that would probably suit a smaller hand. If you wished to join a student or amateur orchestra violas are in short supply and you may be accepted at a slightly lower standard than say a first violin. There has to be a deterrent, of course......a new clef.
My own preference is violin for several reason, one being the amount of music available and, you can always play the vocal line of songbooks. I play (played is perhaps better!) the piano but it never quite caught my enthusiasm like the violin. I love the fact that I can just take it wherever I like (and in my household I'm sometimes incarcerated in the breakfast room with a three-pronged ebony mute!!!
I shall try a viola once I get a few things sorted out on the violin.
Best of luck.
Peggy.