Learning to play violin or viola can be very rewarding. Once you learn to play one it is actually simple to become proficient on the other instrument as well. There are just a few minor differences one should know when switching from violin to viola.
The viola is just a bit larger than the violin; the notes are a little farther apart on the fingerboard, so one has to stretch the hand a tad more. However, the finger pattern and positions are the same. The bow is slightly heavier for the viola but you still use the same bow hold.
Perhaps the biggest obstacle one might face when switching from violin to viola is changing the clef. Most people have some familiarity with the treble (or G) clef, which the violin uses, while the viola uses the alto (or C) clef. Some people claim the alto clef is hard to read; actually it is no more difficult to read then the G clef. Like anything in music, you have to practice to master it.
I highly recommend that whatever instrument you decide to play you learn aspects of the other and become a “switch hitter”. The big benefit is you will always be in demand, especially on the viola. Orchestras and chamber ensembles always need viola players, while finding a gig as a violinist may be difficult from time to time, as there always seems to be a glut of violin players, so if you can play both you will always have a place to play.
Both instruments have a rich repertoire. While there is less viola music if you like to play Baroque and Classical Period works, the composers of the Romantic period, such as Berlioz, Dvorak, Brahms, Wagner, Mahler, and Richard Strauss wrote wonderful parts for the viola. During the 20th Century up to the present such composers such as Hindemith, Elgar, Walton, Bartok and Penderecki wrote many beautiful works for the viola. One of the greatest musicians today, Pinchas Zukerman, switches back and forth from both instruments on a regular basis. Whatever instrument you pick to learn, you will gain years of enjoyment, but if you learn both you will more than double your pleasure.