APNG is lossless and large, and many apps show only its first frame. MP4 plays on every phone, browser, TV and social network. APNG is at home in animated stickers and graphics that need real transparency; MP4 is the usual choice for sharing and uploading video anywhere.
The defaults suit most files. Under Options you can set the frame rate, set the width and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because APNG is not handled by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB.