While a higher PPI is always better in theory, before making any purchase, you should also know that the human eye is probably not able to detect any difference in PPI beyond a certain limit. However, when it comes to determining the exact limit at which the human eye stops seeing more detail on the screen and pixel density stops mattering, this is a matter of debate.
So far, neither researchers nor ordinary people can give you a straight answer and say that “pixel density becomes meaningless after, say, 500 PPI.” However, according to Roger Bourne’s Fundamentals france telegram data of Digital Imaging in Medicine , the magic number would be 400 PPI if you look at an image found at a distance of about 16 inches or 40 cm from your eyes. This is because the human eye has 400 sensory cells per millimeter of retinal surface.
400 to 450 PPI is good for mobile phones.
This is probably also the main reason why modern smartphones, for example, no longer place much emphasis on pixel density and high resolutions (such as QHD and 4K). While this was the trend a couple of years ago, manufacturers currently prefer to stick to FHD+ resolution for their mobile phone displays.