It depends on if you have long hands and long fingers or not, violin is smaller than a viola so if your small you could play the violin instead, it also depend if you want to go to orchestras when you've passed grade 6. Violin is usually the melody and viola is sub music. Viola and violin has the same difficulty, but if you have some background knowledge of the treble clef, violin would be better because viola uses alto clef, in higher standards, viola would also use the treble clef which is quite confusing.