MXF is a professional exchange format that phones, browsers and consumer editors do not open. MKV can carry several audio and subtitle tracks in one file. An MXF file usually holds MPEG-2 (XDCAM), AVC-Intra, DNxHD, ProRes, DV and more. MXF is at home in broadcast and professional camera footage; MKV is the usual choice for films with several audio and subtitle tracks.
The defaults suit most files. Under Options you can lower the resolution, trade quality against file size, remove the sound 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 MXF 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.