File size
MP4 with H.264 produces the smallest mainstream files at a given quality.
MOV with the same H.264 codec is roughly the same size as MP4 — the container adds very little overhead.
AVI files are typically larger, both because the container is less efficient and because AVI is often paired with older codecs.
Quality
Quality is determined by the codec and bitrate, not the container. An MP4 and a MOV with the same H.264 settings look identical.
AVI's quality depends entirely on the codec inside — modern H.264-in-AVI looks great, but legacy DivX or uncompressed AVI clips can look much worse or take far more space.
Compatibility
MP4 plays on everything: browsers, phones, smart TVs, social platforms.
MOV plays on every Apple device and on Windows via QuickTime or modern media players. Some platforms reject MOV uploads.
AVI plays everywhere on desktop but is less consistent on mobile and web.
When to use each
Sharing, uploading, or embedding: MP4.
Editing in Final Cut, iMovie, or Apple-first pipelines: MOV.
Legacy tools, Windows-only workflows, or archive compatibility: AVI.