I did some testing today and I must admit that I was wrong. I used Adobe Audition to convert A No Doubt song to MP3 @ 320, a fast 128kbit and a slow one that should be better. Also I converted the song to Atrac 132 66 and 105.(I also converted it to atrac+48,128,192 but sonicstage crashed so I can't play it back)
I was very surprised that the sound from atrac 132 wasn't that different from 320kbit mp3! The track Excuse me mr. starts with a guitar panned almost on one side. One the 132atrac and the 320kbit mp3 the unused channel was still completely clear. (like you only want to hear beautiful guitar reverb and studio room feedback instead of strange noize) On the 128kbit mp3s (and 105,66atrac) it wasn't. However the higher frequencies of the drums sound better on the 128mp3 than on the 132kbit atrac file.
So to save space 132 atrac is great and you only lose a little bit in the high frequencies. mp3 is more unreliable what it does with extreme panning.
I'd like to do more tests but sonicstage crashed and won't work, so I'm afraid I'm done with MD. today it deleted 5 md's before sonicstage crashed.