Why convert MP3 to AAC
AAC was designed as the successor of MP3 and handles the same bitrate more efficiently, especially below 160 kbps. Some devices and apps are built around it: certain car head units, set-top boxes, broadcasting tools and older Android players list AAC as their preferred format. A raw .aac file is also what some video and streaming tools expect when they ask for an audio track without a container.
If you mainly want your music to work well on an iPhone or in Apple Music, consider MP3 to M4A instead. M4A holds the same AAC audio but adds tags and accurate seeking, which a raw .aac file lacks.
Quality: a second encode, not an upgrade
Your MP3 has already been compressed once. Encoding it to AAC compresses it again, and the new encoder cannot see what the first one threw away. AAC efficiency does not help here: a 128 kbps MP3 turned into 96 kbps AAC will sound slightly worse than the MP3, not the same. The rule is simple: choose an AAC bitrate equal to or higher than the MP3 bitrate.
- MP3 at 128 kbps: use AAC at 128 or 160 kbps.
- MP3 at 192 kbps: use AAC at 192 kbps.
- MP3 at 320 kbps: use AAC at 256 kbps; the extra headroom is not audible.
Size and compatibility
At the same bitrate, AAC and MP3 files are the same size: 128 kbps is about 0.96 MB per minute, 192 kbps about 1.44 MB. So this conversion saves space only if you lower the bitrate, which costs quality. Raw AAC plays in VLC, most phones and modern browsers, but Windows Media Player before Windows 10 and many web upload forms reject it.
What is not carried over
The ADTS format used for .aac files has no place for metadata. Title, artist, album and cover art from the ID3 tags are dropped. Some players also show an approximate duration for raw AAC, or jump slowly when you seek. If you only need a smaller MP3, Compress MP3 is the simpler route.