Should I Encode?

NO!

you cant get better quality by going from 1 format to another, you will only decrease the quality

AAC has better quality at a given bitrate, if anything you would want to re-rip your music from CDs to AAC

Just leave them as mp3s

 

Technically then, assuming the quality wasn't THAT bad you could just encode the mp3s to a lower bitrate AAC file to save space.

 

yes you could but the mp3 and aac codecs are quite different, and thus will discard different parts of the audio, so you get stuff discarded by the mp3 encoding and then stuff discarded by the aac encoding, i'm sure there must be a better way of explaning this

 

You are correct in what you are saying, and it is tough to describe. Here's the 'rub' though: If the mp3's are high bitrate (say 192) and you have less than optimal hearing, converting to 128VBR AAC may not have a noticable loss of quaility to you but WILL save you some space on your iPod.

I would recommend that, if you either have the original CD or a good backup of the 'original' mp3 files, and if the mp3's are 160-256 kbps, and if you are concerned about space NOT quality, that you try converting a few tracks to 96VBR and 128VBR. Go to a quiet place and listen to them to see how they sound to you. If the sound is good to you then go ahead and convert them all for space saving reasons. Just be sure to keep a backup handy if you ever want to the higher quality file.

Otherwise, if they sound bad, leave them be and think about reripping from CD if you really need to save space. I am just finishing converting all my music to 96kbps VBR AAC from 160+ mp3. I rip all my new stuff in 96VBR now as well. I have significant hearing damage and can't tell the difference, so I get a "free 20%" space increase on my iPod. I have all the high bittrate mp3s on DVD-R's, plus the source CDs, so I can always go back to them.

The other bonus is that my music sounds crappy to alot of people, so they don't want to borrow my iPod. ;-)