Every bass guitar has its own unique sound; from the dark thump of a Hofner 500/1 to the brash tones of a Music Man Stingray, every bass is different. A bass's sound is affected by what the bass is made with: wood, pickups, electronics, etc. The key way of defining a bass's tone, however, are the strings.
There are two types of strings for bass guitars: flatwounds and roundwounds. Flatwound strings get their name from the flat wire wrapped around the core of the string. Roundwound strings get their name from the round wire wrapped around the core of the string.
Flatwound strings are considered to be "traditional" strings because they were the standard for the modern basses in the '60s. Flatwounds have a darker, warmer, vintage tone that was very popular in the '60s and '70s. A prime example of the sound of flatwounds is the Hofner 500/1 used by Paul McCartney in the early recordings of The Beatles.
Flatwounds are good for beginner bass players because of their smooth feel. They are also more durable than roundwound strings, with the drawback being that they are not are flexible and are more expensive than roundwounds.
Roundwound strings were popularized in the '80s and are much more common today than flatwounds. Roundwounds are naturally brighter sounding and not as muted as flatwounds. A prime example of the sound of roundwounds is the Music Man Stingray used by Flea in early recordings of the Red Hot Chili Peppers.
Roundwound strings are very well known for their versatility; roundwounds can be made to sound like flatwounds, but flatwounds can't be made to sound like roundwounds. Roundwounds are also more flexible than flatwounds, meaning that you can effectively use more playing techniques, such as slap and pop, on roundwounds than on flatwounds. On top of all that, roundwounds are typically cheaper.
All in all, I suggest using roundwounds over flatwounds, unless you are only wanting to play vintage sounding music. Both strings sound very good, but the versatility of roundwound strings is fantastic. But don't take my word for it, try them out for yourselves and see what you like the best.
