I use both, but don't really feel like there is a useful comparison.

My lightly patched st /feels/ better IME, but it doesn't have anywhere near the same featureset. kitty has its fancier font support and cool kittens which make the comparison very uneven.

I limit my kitty use to a single long running instance as my vim terminal, taking advantage of its far better rendering for pretty symbols and inline images via the icat kitten. For everything else I find st/urxvt far more useful, in spite of the occasional rendering errors and font selection problems. Even going so far as to fiddle with sxiv's Xembed support to weakly imitate kitty's icat from time to time.