AAC is MP3's better-trained successor: cleaner sound from the same number of bits. It was born in 1997, an MPEG all-star team: Fraunhofer, Dolby, Sony, AT&T and Nokia, and its real weak spot is older or stubborn hardware that only ever learned MP3.
AVI is the veteran of desktop video: dependable in 1995, cranky about everything invented since. Audio Video Interleave: an early container that predates streaming and most codecs you'd use today. That makes it the right destination when what you need is opening old archives and humoring legacy Windows tools.