Raffaele,
There are certain classic portrait conventions (not painting bigger than life, placement of the subject in the canvas, not lopping off fingers by painting them out of the frame, etc.), but absolutely nothing that says a head and shoulders is the lesser cousin to a 3/4 portrait. Ormond's
John Singer Sargent, the Early Portraits is filled with exquisite, moving, and defiantly classic portraits, and they are for the most part head and shoulders portraits.
A head and shoulders portrait will place a greater emphasis on the features of the sitter, their expression and thoughts. The subtle lift of an eyebrow, down cast eyes or the flicker of amusement animates and explains who this person is. In a 3/4 portrait, you can work with body language, props and atmosphere to create that recognition.
(If limited to a 24" x 20" canvas, I personally would do the head and shoulders).
Peggy