x265 is superior technically, but not all x265 releases are superior to all x264 releases. A high-bitrate x264 release (e.g., 10GB for a 1-hour episode) is often visually superior to a starved x265 release (e.g., 2GB for the same episode). Therefore, Sonarr must be configured to prefer x265, but filter out low-quality garbage.
Once the format is created, you must give it a "score" so Sonarr knows to prioritize it over other releases of the same quality (e.g., preferring a 1080p x265 over a 1080p x264). sonarr prefer x265
Smaller file sizes require less bandwidth to download and stream remotely. x265 is superior technically, but not all x265