I notice the atitude indicator displayed in the PFD is either squared-shape or truncated-circle-shaped.
A good example of the square shape is the B747-400, It looks like a square whose corner are a bit rounded:
A good example of the circle-shaped is the SAAB2000 or the A340:
They look like a circle whose both right and left sides are truncated to display rules (speed and altitude).
In either case, the information displayed are the same (pitch, bank angle, perhaps flight director,...) and are displayed in a very similar manner (including the colors). Those 2 designs exists for years, so there may not be advantages of one over the other. Why one is preferred over the other for some manufacturer, and the other one for other manufacturer? What criteria are taken into account when choosing this shape?