MP4 is the compatibility-first choice
MP4 is supported across a very broad range of consumer software, devices and publishing workflows. When you need to send a video to someone without controlling their playback environment, MP4 is often the simpler choice.
WebM is closely associated with web delivery
WebM was designed for media on the web and is supported by modern browsers. It can be useful for browser-first assets and workflows that specifically call for WebM-compatible codecs.
Container is only part of the story
File size and visual quality depend heavily on codec, bitrate, resolution, frame rate and encoder settings. Converting MP4 to WebM does not guarantee a smaller video, and converting WebM to MP4 does not automatically improve compatibility with every application.