Compression
AVIF typically lands 20-30% below WebP at matched visual quality, and the gap widens at low bitrates — AVIF holds together at aggressive settings where WebP starts to smear.
Colour and dynamic range
AVIF supports 10- and 12-bit depth, HDR and wide gamut. WebP is 8-bit sRGB and nothing more. If you are publishing HDR photography, that settles it.
Encode cost
AVIF is dramatically slower to encode — often an order of magnitude. In a build step processing thousands of images, or a CDN transforming on the fly, this is a real operational cost.
Support
WebP is supported by every browser in current use and, increasingly, desktop software. AVIF has Chrome 85+, Firefox 93+, Safari 16.4+ and Edge 121+ — broad, but younger, and desktop application support lags further behind WebP's.
What most teams actually do
Serve both, in order: an AVIF source, a WebP source, and a JPG fallback, inside a element. Each browser takes the best format it understands and nothing breaks.