A:Answer The primary difference is that the Z9D has full array local dimming (FALD), with far more individual dimming zones (approximately 650) than any other TV in their lineup, and more than any other LED TV from any manufacturer, with the possible exception of one model from Panasonic. This huge number of zones permits the TV to rival (though not quite equal) OLED in its black levels, while retaining the brightness necessary for good HDR that OLEDs lack. When I compared it to OLEDs and other LEDs the Z9D was overall the best TV I saw by a mile. It's not perfect, no TV is, but it is far and away the best compromise between the competing strengths of OLED and LED.
Just search any of the owners threads online and you will quickly see why it has gained legendary status. There's just nothing else quite like it out there, which is why even though it is a 2016 model Sony retained it as a flagship through 2017. Better yet, go see one and I guarantee you will be stunned.