The difference between 9s and 10s on a Stratocaster guitar refers to the gauge of the strings. The gauge of a guitar string refers to the thickness of the string and is measured in thousandths of an inch.
9s are considered a lighter gauge of strings and are often preferred by guitar players who like a “tighter” feeling on the fretboard and a more delicate touch.
They also tend to bend a little easier and have a slightly brighter tone. 9s are more comfortable to play, especially for players with smaller hands and they are suitable for players who play with a lot of vibrato and bending.
10s are considered a heavier gauge of strings and are often preferred by guitar players who like a “looser” feeling on the fretboard and a more robust touch.
They also tend to have a little more tension and a slightly warmer tone. 10s are more suitable for players who prefer a more aggressive playing style and for players who tune down to lower tunings.
Ultimately, the choice between 9s and 10s comes down to personal preference and playing style. Some players may find that they prefer the feel of 9s, while others may prefer the tone and tension of 10s.
Experimenting with different gauges can help you find the right set of strings for your needs.