No difference.
Someone at TGP reckons that the Mains Out cabs OFF sounds much better than Monitor out Cabs OFF...there is no difference, both activate the Cab Driver.
CK confirms this in his post at TGP, but they don't seem to take any notice.
"There is two ways to bypass the cabinet section and activate what we
CabDriver: use "MonitorCabOff" to globally bypass the virtual cab for
the mono monitor output only, while the full studio sound is still
delivered.
Or switch off the cabinet section directly. The latter has no sonic
benefit, it delivers the same sound, but stereo to the main outs as
well.
"