There is a 3 year difference between them, I would choose U2415. While visually you might not see much difference on image quality there are some minor imporvements. Additionally the ports and connections differ between the monitors.
More details can be seen here...
For some time I have serious issues with my current 4 years+ old TN LG panel when reading text (which is kind of blurry, should have changed it years ago), causing eye strain, lowering brightness used to help but last month this is not helping anymore. Also lowering blue light with sofware does...