Great question - I can answer with the experience that my Father had during his route days.

Gottlieb EMs - Ruled the late 60's and 70's. Late 60's was Williams time and early 70's. Bally's were not as reliable. Generally speaking, Gottliebs were more reliable and were definitely the prefferred EM

Bally SS - Ruled the late 70's and early 80s. Stern were ok, but Bally had no competition. Williams SYS4/6 were unreliable and were poor earners anyway. Gottlieb had lost their way and system ones were hopeless. Dad used to make shelving out of parted out system 1 and Williams machines.

I know Dad stuck to Gottlieb EMs and Bally SS in the 70's. I recall dropping 20 - 30 system one and Williams sys 4/6 to the tip. I was only a child, but I remember it like yesterday.