My MP3 music loop works fine in Unity 2.5: when it ends and restarts, it smoothly keeps the beat and you don't even notice it's a loop. I'm testing the same game with Unity 2.6, and the same music loop now has 1/4 second of silence between the end of the MP3 track and the re-beginning of it. How can I get rid of that gap? My audio editing tools are limited to Garage Band and iTunes (sadly).
Unity doesn't support MP3 (except on the iPhone). I don't know why the behavior is different between 2.5 and 2.6, but your two actual options, uncompressed and Ogg Vorbis, have looped fine in all recent versions of Unity I've used (other than iPhone!). Garageband should be perfectly suitable for editing your audio. My advice is to never import compressed music to begin with, if you need a clean loop. Extra silence likes to creep in at the end of files when they get compressed.
Let us know if this link provides enough info to fix your problem. It's essentially the same situation.
EDIT: I tried what Jessy suggested below (unchecking the "Compressed" box in GarageBand's Share dialog to produce an AIFF), and the resulting file looped perfectly in Unity. Thanks, Jessy!
I recently ran into the same thing. I ended up using a wav instead of an mp3, but even then there was a slight pause when the loop repeated. I had to shave a small piece (equivalent to a 32nd note at 120bpm) off the end of the loop, and then it sounded fine. This was possible because there was a small bit of silence at the end of the loop.
Primitive, but it worked.
I couldn't figure out how to produce a wav in GarageBand, so I used Switch Plus to do the conversion.