At both algorithms' highest levels, xz seems to be on average a few percent better at compression ratio, but zstd is a bit faster at compression and much much faster at decompression. So if your goal is to compress as much as possible without regard to speed at all, xz -9 is better, but if you want compression that is almost as good but faster, zstd --long -19 is the way to go
At the lower compression presets, zstd is both faster and compresses better