FLAC should always sound better than AAC, bearing in mind I’m the guy who invented the biggest part of AAC.
AAC is better than any of the other codecs out there, as far as I know, still IF YOU HAVE A GOOD ENCODER.
But, if you can get lossless, get lossless. Geeeze, louise. I’ve been saying this about codecs (many of which I invented) since 1989 if not before that.
If the lossless sounds worse, something’s wrong with the input. EOF.