I use the sierra single antena...reception for cb and factory radio is very good. The sierra dble antena is just for looks...second antena is a "dummy" antenna and I didnt want to have to drill holes or run unneccesaty equipment. sierra single antena just screws on and no drilling.
I have the Sierra also. Performance is good. I missed the AM reception and put a stock Honda on the other side. ( some modification necessary)
Sierra warns against this because it affects the CB performance. I haven't noticed any CB performance drop with the second antenna installed, but the AM/FM is much improved. You also don't have to answer the question " Do you have a CB" at group rides.
I put the Sierra on mine to avoid having to have the nerdish looking second antenna and more to avoid drilling my plastic and routing lines. The Sierra is plug and play. Sure works better than it did before, except I would not know about AM radio, don't use it at all -- why do they even bother putting AM radio's out these days -- maybe that is just true around here as reception is difficult in our terrain.