MP4 is the world's default video: the file that plays before anyone has to think about it. It was born in 2001, the MPEG standards group, built on Apple's QuickTime design, and its real weak spot is very little day to day: as a container its quality depends on the codec inside.
AAC is MP3's better-trained successor: cleaner sound from the same number of bits. Advanced Audio Coding: the default soundtrack of YouTube, iTunes and most MP4 video. That makes it the right destination when what you need is streaming-grade sound at friendly file sizes.