Why AAC generally sounds better than MP3 at the same bitrate
AAC (Advanced Audio Coding) was designed as MP3's technical successor, using more modern compression techniques that generally extract more perceptual quality out of the same number of bits — at equivalent bitrates, most listeners find AAC audio indistinguishable from or slightly better than MP3, particularly at lower bitrates where MP3's older compression approach shows its limitations more clearly.
This is exactly why Apple adopted AAC as its default format for iTunes/Apple Music and why YouTube, most streaming services, and Android's default codecs all rely on AAC rather than MP3 for anything encoded today — it's simply a more efficient codec for the same amount of storage or bandwidth.
Where AAC files actually come from
Music purchased or downloaded from Apple's ecosystem, audio embedded in most modern video files (MP4 containers typically carry AAC audio tracks), and audio extracted from YouTube or similar platforms is very often already AAC, whether or not the file extension makes that obvious — this is why converting “from AAC” frequently comes up when someone is trying to extract or repurpose audio that was never explicitly labeled as an AAC file to begin with.
Compatibility caught up years ago
AAC's one historical disadvantage — being newer and less universally supported than MP3 — has largely disappeared over the past decade. Virtually every modern phone, computer, car system, and media player now decodes AAC without issue, which removes what used to be the main reason to prefer MP3 for pure compatibility. The remaining edge cases are older or very budget hardware that hasn't been updated in a long time.
Choosing AAC vs MP3 today
For any new conversion where you're not constrained by a specific legacy device, AAC is generally the better default: equal or better quality at the same bitrate, with compatibility that's now close enough to MP3's for almost all practical purposes. MP3 still earns its place specifically for the rare cases involving genuinely old hardware or software that predates AAC support.