Having had both the ZF9 and the S23 for a relatively long time (esp. the ZF9), I have compared them head-to-head in every way possible.
In terms of size, the ZF9 is clearly much handier. They are equally tall, but the Asus phone is almost 3mm narrower, which is quite substantial.
Also, if you do not use a case, the Asus will be much less slippery and less prone to serious damage if it falls. That said, I appreciate the thinner S23 and I like its small, unobtrusive camera bumps. I deeply dislike the ZF9's bulging cameras - though aesthetically they look nice, I admit.
In terms of sheer software customizations, both offer excellent options - I would say they are comparable in this respect. But Asus wins on stock Android, if this is your cup of tea (it is mine); it's also friendlier to Nova Launcher (I have not tried other third-party launchers). Still, I have to admit Samsung has made a big step forward with the latest OneUIs., though it's still bloated.
Audio-wise, Asus wins hands down with the 3.5" jack and the BT codecs. The speakers are absolutely comparable - and both phones are very good.
Where the S23 has a massive upper hand is in two departments: screen brightness (Samsung is amazing in direct sunlight); and camera (though both are way too cheerful with the colors). The S23 does far better on zoom given its tele and, most importantly, is much more reliable. Asus does rather poorly with pets and kids; and has a problem with red or deep purple flowers (totally messes up their textures if in bright sunlight).
A hard pick. I have stayed with the ZF9 - but that's because I always have my Pixel 4 and 5 at hand.
