Jack(Lazyrider) installed a Stebel airhorn on his bike a few weeks ago and it is loud! Today, since I am electrically challenged, he helped me install a couple of Fiamm Freeway Blasters on my bike. They are LOUD. The low frequency(according to Fiamm) carries a longer distance. The high frequency is loud in the immediate area.
Although we haven't had a 'honk-off', I think both the Stebel and Fiamm are an order of magnitude improvement over the stock horns and will get most any drivers attention. Subjectively, they seem equally loud.
The Stebel is neater looking. The Fiamm's are normal plastic shells and can be mounted using the stock bolt. The Fiamm's cost about $20 with shipping and $5 for a relay. I'm not sure exactly what the Stebel cost but it was a little more.
You can't go wrong with either horn and would be a great safety improvement for any bike.
