What dictates the shape of the attitude indicator display on the PFD?
A circle makes sense because a physical attitude indicator is round. (A physical one has to be round because it consists of a ball moving behind a window.) It makes the screen display look more like the instrument it replaces, which is good for recognition and familiarity, and makes it easier to judge things like bank angles.

A rectangle makes sense because it has to fit on a rectangular screen. You can tile rectangular instruments/displays on a screen efficiently, with no wasted space between them, as your example shows with the tapes. Rounding the corners delineates the edges of the display better than just having abutting rectangles.

The full process by which physical cockpit instruments and glass-cockpit displays are designed and laid out is too long to explain here. The main focus is on making it hard to make mistakes, and after that to make it quick to see the most important information. To support this, each instrument has to be easy to identify and to read correctly. Despite the shape difference, the two designs look very similar. Even if you've never flown any of these aircraft before, you'd immediately recognise it as an attitude indicator, and you wouldn't confuse it with a different instrument.