With the model II Sony added an accessory shoe that would accommodate an auxiliary EVF, the same one used on the RX1. With the model III the accessory shoe is gone and an SVGA OLED EVF with 1.44M dots is now a built-in pop-up, all this while keeping the camera the same size -- amazing.
The appearance and size of the RX100 has not changed through its three iterations -- a minor miracle of Sony engineering..
The second big change made to the new model is the redesign of the Zeiss zoom lens to conform to a wider 24mm that now caps out at 70mm with a very bright variable aperture of f/1.8-2.8. Sad to lose the extended 100mm equivalent range of the prior models, but the wider 24mm increase over the prior 28mm angle plus gain of approximately 1-stop of lens speed at 70mm is a major plus.
The dexterity of the flip screen has been increased considerably. It can now flip upward so you can see yourself from the front of the camera -- a boon to Instagram shooters. It does illustrate that this camera has appeal to all levels of use, from professional images grabbed by having an inconspicuous, easy-to-pack camera always with you to personal and social media casual shooting. This has been one of the chief features I have liked about this camera -- it is the portable enough to always have with you. More than once I have had to rely on it for some serious, opportunistic shooting.
The cost of the new model has increased to $798, but when you consider the very high cost of the former accessory EVF, having one built-in for only a $100 increase in price is a real saving.