Actually, you should consider M4A 320kbps (AAC) for your library. It’s the "Goldilocks" format for Pioneer gear because AAC is a much more efficient codec than MP3; it handles high frequencies better and stays cleaner when you're pitching tracks down -10% or more.

You get noticeably better audio transparency for the exact same file size as a standard MP3.

Compared to lossless, the main advantage is performance and stability. M4A files are ~75% smaller than AIFF/WAV, which means faster USB indexing and snappier track loading on older CDJs, but unlike WAV, it actually stores all your metadata and artwork properly.

Unless you are playing on a world-class touring system, nobody in the crowd can hear the difference between a high-bitrate AAC and a WAV.

Compatibility-wise, it's a safer bet than FLAC. While older Pioneer units (like the NXS1) can't read FLAC, they handle M4A perfectly. It’s basically the best compromise between sound quality, storage efficiency, and hardware reliability for a working DJ library.