Why are sash windows best for aesthetics, well mainly because of the Palladian theory.
The human eye has the ability to read lines and angles extremely well, to see if something is balanced or unbalanced. Palladio understood this more than most.
He drew an arc from the width of a window, which equalised the shape of a window and meant that the glazing bars were equalised and the finished design, well balanced. Sadly, many modern window designers have lost this principal, most window producers or installing companies provide unsympathetic products, which have totally destroyed any semblance of his theory.
Most box sash windows were traditionally fitted behind the outside brick or stone, so that the bulk of the frame material was hidden from view. This design came after the Great Fire to prevent the spread of flames whilst allowing the maximum daylight into the rooms.
We know about these attributes, our installers will conceal most of the frame too, rather than installing products that are unbalanced.