MP3 is the format that taught music to travel: small, scrappy, and welcome absolutely everywhere. It was born in 1993, Fraunhofer Institute, Germany, and its real weak spot is holding fine detail at low bitrates: it stays small by quietly discarding sound.
MKV is the format that hoards: video, a dozen audio tracks and every subtitle, all in one box. The Matroska container: it will hold practically any codec ever made. That makes it the right destination when what you need is high-quality rips and archives with multiple audio and subtitle tracks.