My quick test:
Karl Bohm, Wiener Philharmoniker / Mozart: Requiem d-moll, KV.626
Nero 1.5.4.0 -q 0.345 -ignorelength - 96 kbps avg
QAAC v0.47 (QuickTime 7.6.9) -s -V 46 -q 2 - 99 kbps avg (QAAC's resultant audio stream is bit-identical to QTAACENC --tvbr 46 --highest --samplerate keep)
But as for me there are also audible (and visible) diferences in encoding of high frequencies (while the lowpass filter frequencies are nearly equal). I can hear that for QT background noise has a little wider frequency range:
foo_abx 1.3.4 report
foobar2000 v1.1.7
2011/07/02 11:15:26
File A: D:\Моя музыка\Mozart\Mozart - Requiem [Mathis, Hamari, Ochman, Ridderbusch - Karl Bohm] (1990) - DG\03 Tuba mirum_qaac.m4a
File B: D:\Моя музыка\Mozart\Mozart - Requiem [Mathis, Hamari, Ochman, Ridderbusch - Karl Bohm] (1990) - DG\03. Tuba mirum_nero.m4a
11:15:26 : Test started.
11:15:58 : 01/01 50.0%
11:16:03 : 02/02 25.0%
11:16:10 : 03/03 12.5%
11:16:21 : 04/04 6.3%
11:16:27 : 05/05 3.1%
11:16:39 : 06/06 1.6%
11:16:52 : 07/07 0.8%
11:16:55 : 08/08 0.4%
11:16:59 : 09/09 0.2%
11:17:02 : 09/10 1.1%
11:17:06 : 10/11 0.6%
11:17:13 : 11/12 0.3%
11:17:20 : 12/13 0.2%
11:17:32 : 13/14 0.1%
11:17:36 : Test finished.
----------
Total: 13/14 (0.1%)
Here is the spectrogram of two tracks (the first part is for QT, second - for Nero).