Hard for me to tell as with Flac encoders I've used, seems limited to only 16 bit depth (anyone know of Flac 24 bit, and maximum sample rates over 48K?), but Flac does NOT limit encoded high frequency bandwidth and is supposed to be totally lossless NOT throwing away any audio file information.
MP3 seems always to more-or-less affect high frequencies encoded bandwidth and also seems limited to 48K sample rates.
I have used MP3 encoders working directly with 24-to-32bit (float) bit depth files giving 'chance' of more encoded audio details. But as is generally well known, all MP3 encoders, regardless of encoding (to 320 kbps) rates are NOT lossless, so throw away information to reduce file sizes, and Flac does not do this.
No clear winner here if 24 bit depth MP3 encoding has some benefits over 16 bit Flac limitations.