I see.
Why do you use AAC though? Not that I've listened to very much of it, but I think I prefer MP3 in terms of quality to size ratio.
If quality and size are the deciding factors you should go with AAC. A 128 mbps AAC file is notably smaller than a 128 mbps MP3, and if I'm remembering correctly the quality of a 128 mbps AAC is approximately the same as a 192 kbps MP3.
The advantage with using MP3 is that you can use it in pretty much any player, although iPods aren't the only players that support AAC.