"What is the difference between the Rocket Appartamento and the Rocket Espresso Cellini/Giotto Premium Plus"
These machines are actually quite similar but there are differences. Lets go over them.
Insulated Boiler - The Cellini/Giotto Premium Plus contains an insulated boiler where the Appartamento does not. This means the Appartamento will be hot to the touch pretty much everywhere and the life of the machine will be degraded due the internal components being exposed to excessive heat.
PID Temperature Control - The Cellini/Giotto have this capability while the Appartamento does not. PID allows the user to adjust brew temperature to their desired amount making you espresso much more customized.
Power - The Appartamento comes in at 1300 watts where the premium plus models come in at 1350 watts. This means more power to the heating element and a decreased heat up time for the premium plus models.
Price - Appartamento - $1,500. Premium Plus Models - $1,950
Noise - You'll notice a distinct different in noise. The Appartamento is much louder.