For quite some time, I thought the variance between models of Sega Genesis/Megadrive were as simple as “model 1, model 2″ et cetera. I was vaguely aware that the sound chip changed from model to model, but simply thought it “got worse” over time. The answer is far more complex than that, as you can read in this extensive investigation of the subject on Sega-16’s forums.
The number of changes Sega made from model to model, down to the circuitry, is pretty indicative of why Sega would later lose the war against companies with more consistent messages and systems. The conclusion is that you want a “high definition graphics” genesis if you care about sound, and if you’d like the game to boot straight away, skipping the “under license from Sega” screen, you’ll want the one with this FCID.