This post was written just to show you the picture of both machines.
The boxes are near identical externally (the MC400 has a large touchpad just near the screen and the MC600 has additional button and small display instead of the touchpad).
The rest of hardware is mostly identical (the MC600 has more memory due to DOS requirements) except some I/O connectors.
I still haven’t used my MC400 too much because it’s software is somewhat unusual (even the interface and keyboard shortcuts are different from later SIBO machines, like the Psion Series 3). At least it is good for development of non-interactive OPL codes.