Flat wound strings have a smoother, more consistent tone, whereas round wound versions have more sustain and bite. Flat wounds also have less resistance, so they’re good for fast playing. They’re also great if you want a vintage tone – Jimmie Vaughan uses flat wound strings.If you want to shred, it has to be light gauge strings. Say, .009s. Light gauge strings are easier to fret and easier to bend, so lend themselves to playing at speed. Heavy gauge strings are great if you play in a drop tuning, because they retain their tone better at a lower tension. They can also be good for slide playing. The final choice, other than the brand, and that really is a matter of personal preference, is the material the strings are made of Steel is bright and has lots of sustain. Nickel is warmer and great for those vintage tones. You can also buy nickel-plated steel strings, which offer some of the advantages of both steel and nickel, being warmer than steel but having greater bite than nickel. There’s also chrome, used by jazz players and anyone looking for a tone that sounds like T-Bone Walker.