FAAC lower quality per bitrate than MP3lame? Options

[F J Walter]

I have done some personal testing which is ABX style where I put the control and the compressed version into a playlist, hide the playlist window, then I listen to the first one and the second one, then shuffle and see if I can identify whether the next one is the control or the compressed version.

But I have found some strange results and that is that I am able to consistently identify the AAC at -q 120 and above, whereas I can't reliably identify (ABX) the MP3 LAME encodings at q 6 and below. Q 6 is actually a much lower bitrate than -q 120 in FAAC! So if you believe my results, you may conclude that LAME MP3 is more efficient in a perceived quality per bitrate than FAAC, which is AAC-LC!

I am doing the encoding with ffmpeg, using the latest snapshot libraries of libmp3lame and libfaac which I compiled recently. The reason for using ffmpeg is twofold - first, I can encode such that the only variable is the audio codec; the container format is the same. Second, because the whole purpose behind me doing this is figuring out a format to use as audio when encoding videos, (some of which are music videos, so I want good quality).

Is it possible that MP3 is just more transparent to me in general? Or that LAME is a lot more efficient than FAAC? Or am I not hearing artifacts in MP3 because they are masked due to the nature of the source material, and with other material I might have different experiences? The source material is an acoustic band, with a male vocalist, doing a slow ballad - quite a good recording I think.

Command lines (typical)
ffmpeg -y -i testsong.wav -ss 9.5 -t 25 -vn -acodec libfaac -aq 120 out.a120.mp4
ffmpeg -y -i testsong.wav -ss 9.5 -t 25 -vn -acodec libmp3lame -aq 6 out.m6.mp4

1st line generates 435KB file, 2nd line generates 361KB file, plus I generated a control file which copies the raw audio.