waitaminute, the genesis 1 is compatible with the sega cd 1, the genesis 2 is compatible with the sega cd 2 and the genesis 3, is pared down(smaller size, no headphone jack)i thought it was compatible with either the sega cd 1 or 2, but it fitting it together was messier(it wouldnt fit in the tray, the sega cd 1 has a tray to hold a model 1 genesis)
Point being: if you dont want a sega cd, then go ahead and get a model 3, bc the smallest one is the best(takes up less space) but if you really want a sega cd, well then try to get a deal on the sega cd first, then get a genny system that matches your sega cd model(one or 2) That is my suggestion anyways.