This is such a huge topic, I wish I had more time to read every post, but its too much. Honestly I have a lot of things to say about this topic, but my opinion is my opinion (so ill try to stick to the facts). Ill also try to keep this short and sweet.
320 vs Flac
Ok so let me say that this is such a broad thing to compare.. Here are some factors you have to consider when arguing JUST the "320 bitrate vs FLAC"
First off how are you getting the files? Are you ripping them from a CD? What method are you using to rip them. Are you downloading them from random places on the net?
I can tell a big difference in sound quality with certain songs and the biggest differences usually are not from this being flac and that being lossless mp3. You can't always look at the exact statistics on paper, because that isn't always set in stone. On paper FLAC should always sound better because it is "lossless" and the frequency range can go below 20 and above 20k hertz. Let me remind you guys that humans cant hear much more than or less than those numbers, if at all. Its just such a broad topic I could talk about it all night. My main point here is that it is very situational. I for one feel safer with my audio library, mostly consisting of FLAC. Why? Let me talk about that.
I get a lot of my music from the internet, most of it actually. A person that prioritizes lossless music w/ thousands of songs versus someone that does the same thing, but with lossy MP3 files.. The person with the thousands of lossless will win everytime when it comes to quality. Not because every time you click one of his songs, the song is better because its FLAC, but because some of those songs might be 24/96 remastered (which by the way, my argument isnt that 24/96 or 24/192 is better because their higher rates, but because a lot of times those tracks are remastered and engineered to sound better than the original, or some of those are the actual original files before taken to cd or vinyl.) And some of those songs legitimately saved some bit of audible noise that the person with MP3 lost out on, no matter how insignificant. Because when it comes down to it, out of 10,000+ songs, I don't think anyone can disagree that its impossible for one tiny part of a song to be missing or disorient an audible piece of music, BECAUSE its an MP3 (LOSSLESS) file. You just can't argue with that. Now at the beginning of this paragraph I mentioned getting most of my files from the web, and yes that is sort of a gamble which reinforces my case. But honestly I feel the same can go for CD also, and ESPECIALLY for DVD and SACD. For me personally and my ears, I can't tell a difference in most songs between 320 and lossless. Its very hard to distinguish, most of the time impossible. When it comes to below 320, it gets a WHOLE lot easier, 256, 192, I can almost tell everytime... BUT.. (and ill end here)..
I have and always will download and rip to FLAC over MP3 anyday, because like I said, out of terabytes of music.. the person who has aquired almost all LOSSLESS music will have a better hear-able collection of music no matter if it comes down to only a few songs that you can notice. For me it is worth it, its a sense of security, its a good way to get remastered, collectors type stuff as well if we are talking about the internet. So my conclusion is, I think that you can't tell more than you can tell generally, but i feel out of a large collection of music you can tell some, depending on the gear and the persons ears, and that is worth the extra storage it takes and the extra time it takes (to rip and to download etc). In the end its up to the individual, but I think every single FLAC vs 320 argument is completely hard to argue unless you are talking about something very situational, and even then, peoples hearing is different.
This is coming from someone with a fairly high end setup, and someone that puts facts far ahead of opinions (in any scenario).
(Thanks for reading)