Atari? If you asked such a question in an Amiga forum in 1990 you probably would have been accused of trolling. I knew people who had Ataris. STs were usually bougt by the parents, because they wanted to do word processing, or expected their offspring to become like Kasparov (who was in the ADs). We knew that the ST had great midi-software, but none of my friends used them for that and they also didn´t own a color monitor.
A cousin of mine had an Atari 800XL and while it was not much different to a C64 it always felt like a cheap "me too"-product to me - made for people who mainly wanted to play games. In fact I don´t know of any serious application or programming package even today, but I guess this is just my ignorance.