There are several difference between a violin and a viola - some of which I had no clue about until I researched the answer to this question.
Violin vs Viola
- The main difference is the frame. Violins generally have a slightly smaller frame.
- A viola's bow also weighs more. And if you look carefully, the horsehair band on a viola is slightly thicker (because it's made with more individual hairs).
- Although the differences between the two bows might seem minimal, for a musician it means adopting a very different technique.
I think violin bows need to make contact with the string before the beat in order to make a sound on the beat. - Violins tend to play higher-pitched parts.
- As part of an orchestra, you can tell them apart based on where the musicians are sitting.
And to cover the clef part of your question: In general violins tend to read treble-clefs, while violas read alto clef.
I'm guessing that, for anyone hoping to switch from viola to violin (or vice-versa) that mastering reading a different clef and finding the correct notes is the trickiest part!
Here's a video I watched when I was learning the difference between the two instruments. Very useful, I recommend having a look: