The Kaiju has a little extra headroom that the MonAmour does not. Both amps use the same plate choke, output transformer, and shunt regulated driver, but the MonAmour comes with fancier capacitors.
The MonAmour is more flexible with tubes used and is cosmetically very striking. The Kaiju is overall a better value, leaves the upgrade choices to the builder, and has a lower noise floor with the DC filament upgrade.