15 or 20, depending on how much room I have. I'll use the smaller lengths just because the cords can get tangled if im not using the whole length, not because of signal loss. The idea that a longer cord will make a difference in sound is crazy, it would only make a noticeable difference comparing a 10 foot cord to a 50 foot cord. Theres no difference in sound between 10 and 20 at all, as long as the cords are of the same quality.