I'm also biased, as I'm the author of Brotli. With Brotli you get about 5 % more density. Brotli decompresses about 500 MB/s while Zstd decompresses about 700 MB/s. Typical web page is 100 kB, so you need to wait 200 us for decompression. For zstd, you'd only need to wait 140 us.

That 60 us saving comes with a cost: you'll be transferring 5 % more bytes, which can cost you a hundreds of milliseconds. Brotli is also more streaming, so you get your bytes our earlier during the data transfer. This allows for pages to be rendered with partial content and further fetches to be issued earlier.

Zstd supporters have used comparisons against brotli where they compare a small window brotli against large window zstd. This makes it seem like zstd can compete in density, too, but that is just apples to oranges comparisons.