I have done many A-B comparisons some 2 years back and came to conclusion that Flac is better than MP3 320 in all respects. My first preference is Flac and if it is not available then I go for 320. I have also noticed a subtle difference in 16bit and 24bit Flac files but that varies from recording to recording. Now I have come over this highly nuanced listening approach and keep my music in 16bit flac files.
I may be having a detailed comparative review of all formats (very technical stuff) which appeared in Stereophile some years ago. Will post those findings if I find that article.