I just bought a 620 Monster but I'm a great fan of motorcross helmets, was wondering if it would be suitable to wear a motocross helmet or even a dual sports helemt on a Ducati Monster, or should I just stick with the full face.

Wear what you want.  I've got an Airoh DS helmet that is really nice for the price; had to order it from the UK though.  Haven't tried it on the monster. It does fog up when sitting, but clears immediately on moving.

Usually they are less aerodynamic and noisier - at least at higher speed.