Not very. AAC (Advanced Audio Coding) is a "lossy" audio compression codec that is meant to be better than standard MP3 codecs but nowhere near lossless, especially at 128k.
Almost any lossy file removes information to a degree that you'll notice. There's software where you can take the output of each respective file and subract the signal from each other and play back the result. The closer the two formats are to each other the less you'll hear after differencing them. If you subtract a 128 kbs lossy file from a lossless and play back the result you'll hear music especially on symphonic works. The noisier songs you'll hear less difference. I would stay away from lossy if possible.
128 AAC (iTunes) is pretty good. 256 AAC is slightly better and hard to identify against redbook in A/B listening on music. Of course, both are lossy so this is not audiophile quality but it is pretty darn good - say 99% there.
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