Quote from Hambone to me in a different thread. I haven't used them, but I trust Hambone's opinion in this.
Quote:
Originally Posted by Hambone
Flat wound strings are the least flexible and therefore the least sustaining strings available. Flexibility is the key to keeping the string vibrating.
I use and always recommend D'Addario Half Round strings for fretless's because they have the flexibility of a roundwound and the flat surface of a flatwound to keep fingerboard damage to a minimum.