I think I calculated 6 vs 8 a while back. My library is currently just over 14,000 FLAC tracks, all encoded at quality 8. I decided on this because even though the space difference is not that impressive, it will help a little bit. Besides: I went into my library rip this time thinking: "This will be the library I will use for the REST of my life."
Yes, that's an interesting way to put it but I really think that is true. I could not have ripped the songs any better: FLAC 8, secure rip, all 100% rips and hand scanned large cover art.
I'm sure new technology may change things but its a big leap from 128Kb MP3s and no cover art that I had before this round of ripping!