ABS is cheaper than metal, particularly because metal prices are on the rise. Typically, however, if you are finding an extremely low cost metal (close to ABS) unless it’s used, it’s because the metal alloy is weak or it is plated. Keep that in mind. That’s not the same as solid metal.
ABS can typically stand up to rocks (especially important if you are mounting under a vehicle) whereas metal may ding or dent, depending on the size of rock and how fast you are driving.
Metal allows for extra vibration, and hence, a louder horn in general. However, the size of the horn and compressor also contribute to loudness and sound output. So if you are comparing a small metal horn with a small compressor, to a large ABS horn, with a large compressor, there might not be a significant a difference in loudness. Make sense?