QHD has twice as many pixels of FHD. So PPI is also going to be twice as much at the same display size.
Pixels are noticeable on FHD 24'' displays. It's an okaish resolution if one is a casual user. I wouldn't recommend it for designers or programmers. But yeah, if the budget has a ceiling and the space doesn't then I'd buy 3x FHD 24'' used monitors instead of one new QHD 24''.