To be fair, I don't even notice a difference between the 25 Ultra display (M13) and S26 Ultra display (M14). I never use phones on full brightness. I rarely ever have my phone over half brightness because it's already really bright at half brightness. When I go outside, putting the phones in adaptive, already makes them fully visible. In terms of image quality, I think we've been at peak performance for a few years now.
I think the only true advantage to the M14 or any newer panels, would be improvements to power efficiency. Brightness only matters when going outside, and flagship phones for the last 4 years have been plenty bright when going outside. I think, the real advantage to outside viewing will now come from the anti-reflective coating used.
This article sums it up nicely. They all have the same M14 panel, but the Samsung runs away with it for outside viewing.