mp3 or m4a

Both are lossy compression formats. They throw away parts of the audio to achieve small file sizes.
AAC is considered to be more effective in that. 128 kbps AAC's and 160 - 192 kbps MP3's have about the same audio quality.

If you only use your songs on a Mac and an iPod, I would recommend AAC, which results in smaller file sizes at the same audio quality (or better quality at the same file size).
If you use your songs on a PC too and other devices (other than iPod), I would recommend MP3.
Although there is sufficient support for the AAC format, MP3's can be played with virtually every audio application and device.

I would not recommend to convert from AAC to MP3 or vice-versa.
Both formats use different algorithms (calculating methods) and converting will result in loss of audio quality.
If your song are from CDs, you're better of re-ripping them in the desired format.
You can find more information about the formats in these Wikis: AAC; MP3

Hope this helps.

M

17' iMac fp 800 MHz 768 MB RAM Mac OS X (10.4.7) Several ext. HD (backup and data)

The choice is a matter of your ear & what you intend to play the files with. m4a is said to sound better at the same sample rate, for example, 128 kbps. So you can use a smaller AAC (m4a) file to achieve the same quality as a larger mp3 file. That said, I use 192 kbps mp3 just for the flexibility of use that mp3 has, as far as mp3 players, etc. Try importing a familiar song with both at different codecs & rates to see what you like. Here's two views including Apple's:

AAC vs. MP3
iTunes 4: AAC and MP3 Codecs Compared