I used to think it was the difference between the Sharp and Panasonic displays but now I’m not so sure, especially as Panasonic was said to only be supplying 5% of displays to Nintendo, at least initially.
There is a definite difference between the 32 pin and 40 pin GBA models, though, which seems mutually exclusive to display type?
Interestingly I still found the GBA SP (AGS-001) to be more viewable due to better viewing angles of the panel, and less flickering.
Check out this thread for more details: Game Boy Advance with 40 pin LCD display cable has more reflective, viewable display