I bought the sebil for 35 of e-bay for my spirit. It has a great sound and draws 12 amps, so the relay IS needed. The gonzo harness makes them a dream to install.
I put two on my valk and they are not much louder in a pair but you can hear me from all directions. The major reason I choose them was that there is nil wind up time for the compressors, and I can give a quick blip, not very obnoxious, just to let folk know I am there, or I can really lay on the horns and folks will fill their pants. Yesterday, I got to do that for the first time with the valk. Some woman in a Mercedes turned into a mall as I was following her and then proceeded to do a three point turn. As she began to back into me ( and I was on an incline so could not move the beast back up the hill to get out of the way) I gave her a blip. No reaction, she was only inches from my front end when I laid on the horns. She stopped immediately, maybe even took a quick heart pill, but I got by her no probs. Which ever you go with, the loud horns are worth it for our bikes.