Supposedly the 6 FOOT (180 cm)would be better that the 39"(100 cm). From all I have read the focal point (which some have said is harder to set on the 6') needs to be correct and the LNB has to match the type of the dish, Offset or Prime Focus. If you are wanting C band, the 6 foot is nec'y. My biggest, in operation, is a 1M for recording on G19.