MP4 or WebM: which should you keep?
Choose for your destination, and preserve the original whenever possible.
For editing and sharing
An MP4 containing H.264 video and AAC audio is a practical starting point for many editing and sharing workflows. Still test your intended editor and device. The MP4 extension can contain other codecs with different compatibility.
For a website
WebM is useful for web delivery with codecs such as VP9 or AV1. MP4 can also work well on the web. Compare the actual output size and visible quality; there is no universal smallest format for every clip and setting.
For a long-term archive
Keep the highest-quality authorized original, plus a convenient viewing copy if needed. Avoid converting repeatedly between formats. Each lossy conversion can introduce additional artifacts. Add a text note describing the source and export settings.