Both options give you the same recording. They differ in how it is packaged, and the right choice depends entirely on what has to open the file.
M4A is the original AAC track lifted straight out of the video with nothing re-encoded. It is an exact copy of what X published, which makes it both the smallest file and the best-sounding one. Take it if you are listening on a phone, a computer, or anything made in roughly the last fifteen years.
MP3 is a real conversion at a constant 192 kbps. It is larger than the M4A and, strictly speaking, one generation further from the source. It exists because MP3 opens on absolutely everything: old car stereos, cheap MP3 players, ancient software and hardware that has never heard of AAC. If the file has to work on a device you cannot update, take the MP3.
The audio inside an X video is AAC at around 128 kbps. That is the source, and no conversion can add back detail the source never carried.
Encoding to MP3 at 320 kbps would produce a file roughly two-thirds larger than one at 192 kbps and would sound exactly the same, because both are comfortably above the point where the conversion stops being the weakest link. The 128 kbps AAC original is the ceiling either way.
Any converter advertising 320 kbps output from an X video is telling you about its encoder setting, not about the quality you will hear.
An X video is a single MP4 file holding a video track and an audio track side by side. Extracting the audio means separating the two and keeping one, which is a fast, lossless operation: no decoding, no re-encoding, nothing recalculated.
For M4A that is the entire process, which is why it finishes almost as fast as your connection can pull the file. For MP3 the separated track is then decoded and re-encoded, which adds a few seconds of processing on top.
Neither path writes anything to disk on our side. The audio is streamed from X through the extraction and out to your browser as it arrives, and nothing is kept once your download finishes.
On iPhone and iPad, downloads land in the Files app under Downloads. Open Files, tap Browse, then On My iPhone, then Downloads. From there you can move the file into a music app or share it elsewhere.
On Android it goes to the Downloads folder and is usually picked up by your music player automatically within a minute or so.
On Windows, Mac, Linux and ChromeOS it goes to your Downloads folder, where iTunes, Apple Music, VLC and every other player will open it directly.
Any public post containing a video has an audio track that can be pulled out of it. That covers the overwhelming majority of what people want: music clips, interview excerpts, speeches, comedy, sound from short films.
Posts from protected accounts cannot be read, because the media is not publicly available to fetch. The same applies to deleted posts and to anything behind a login.
Spaces and live broadcasts are supported directly. Paste a Space link and its audio saves as an MP3; paste a broadcast and you get the video at every resolution it is being streamed in. Neither needs an account.
Saving a clip to listen to offline, keeping a copy of something you posted yourself, or quoting a short excerpt under fair use are all ordinary uses that raise no issue.
Music is where care is needed. A song posted to X is still owned by whoever wrote and recorded it, and downloading the audio does not change that. Using it in a video you publish, or redistributing it, is a copyright matter regardless of where the file came from.
This tool only ever reads what is already public, adds no watermark, and keeps no copy of anything after your download completes.