MKIII is thinner as it does not transport the display information.
MKII is the thick cable and is more expensive. It is necessary when you have speakers with display (Penta, Beolab 4500) or speakers with ICEPower amplifiers.
If you are going to do some building work to hide them, or if you plan to upgrade later to newer speakers get the MKII otherwise the MKIII is enough.
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