Ok, the Atari ST may have been cheaper than the Amiga 500, but to make music with it, you had to connect a MIDI keyboard/synthesizer/etc., whereas with the Amiga you could get by just with its built-in capabilities. This is true for the original STs, apparently the "STE" models released in 1989 also had the ability to play sampled audio, but I don't know if that was ever used as intensively as on the Amiga...