Why such different results? do the two applications really work that differently, when using more or less the same arguments?
I assumed since they both use quicktime that it would be similar results, just different methods of getting the job done.
Some of the tagging is different for the container, and a lot of that I dont even understand, but there is an obvious difference in the average bitrate for the audio stream between the two.
Both qtaacenc and qaac use a range of 0-127 for tvbr, so I am a little confused...
example:
E:\AviSynth\Wavi bd.avs - | qtaacenc --tvbr 90 --highest --samplerate keep --ignorelength - qtaacenc.m4a
E:\AviSynth\Wavi bd.avs - | qaac --tvbr 90 --quality 2 --rate keep --ignorelength - -o qaac.m4a
cmd window:

qaac encoded m4a mediainfo:
General
Complete name : E:\AviSynth\qaac.m4a
Format : MPEG-4
Format profile : Apple audio with iTunes info
Codec ID : M4A
File size : 1.17 MiB
Duration : 33s 259ms
Overall bit rate mode : Variable
Overall bit rate : 295 Kbps
Encoded date : UTC 2011-10-23 03:57:01
Tagged date : UTC 2011-10-23 03:57:05
Writing application : qaac 0.91, QuickTime 7.7.0, MPEG-4 AAC Encoder 1.7.1, Variable Bit Rate q91, Best
Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 33s 259ms
Bit rate mode : Variable
Bit rate : 288 Kbps
Maximum bit rate : 355 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 1.16 MiB (99%)
Encoded date : UTC 2011-10-23 03:57:01
Tagged date : UTC 2011-10-23 03:57:05
qtaacenc encoded m4a mediainfo:
General
Complete name : E:\AviSynth\qtaacenc.m4a
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42
File size : 1.62 MiB
Duration : 33s 259ms
Overall bit rate mode : Variable
Overall bit rate : 409 Kbps
Encoded date : UTC 2011-10-23 04:56:58
Tagged date : UTC 2011-10-23 04:56:58
Writing application : qtaacenc 20110816, QuickTime 7.7.0, True VBR Quality 90
Encoding Params : (Binary)
Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 33s 259ms
Bit rate mode : Variable
Bit rate : 407 Kbps
Maximum bit rate : 428 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 1.61 MiB (99%)
Language : English
Encoded date : UTC 2011-10-23 04:56:58
Tagged date : UTC 2011-10-23 04:56:58