There are a lot of choices in violin and viola strings.
Usually string types are divided into 3 groups: steel, synthetic and gut strings.
The different materials give the strings different properties: Playing in time, longevity, durability, response, and of course sound characteristics.
The first and maybe most important thing is to realize that a good set of strings cannot make a lousy instrument sound like an expensive instrument.
Strings are more a tool for bringing out the tone that is in the instrument they are put on and help balancing out slight issues regarding volume, egality or sound.
Secondly you have to find the price range that fits you. The most pricy strings are not necessarily the best for your instrument. On the other hand the cheapest strings might not be either. Be aware that the cheapest strings often do not have the durability of more expensive sets.
Step three is to find out what kind of sound/feeling you are going for. The string makers use words like: Focused, responsive, brilliant, full, round, warm etc.
Of course you cannot rely on all the positively loaded words, but they can give you a hint to what the manufacturer expects this string to sound like on a good instrument.
Many brands of strings have different tensions or gauges within each line, making the balancing procedure easier.
- For instance I have a nice chinese fiddle with a good bright response on the E A D -strings but the G was a little lacking in power. I changed the G from a medium gauge into a heavy gauge and that gave the right balance. The strings are from the same brand and the same line.
- On an old Italic violin I liked the sound of gut strings especially on G and D but with a gut A the transition to the steel E was just too far. So I chose a steel A and E from one brand and G and D from another.
- My german viola has a mellow sound so I tried to counteract this a bit by using bright strings, which also gives the instrument more penetration.