There's no answer to that, really. It depends on the person. You might try encoding some tracks at various bitrates and see whether you can tell any difference. I personally would go with a 160 AAC file. I don't think I can really tell the difference between 128 AAC and 160, but I thought I would make it a bit higher since I could (and this would also give me more leeway if I wanted to re-encode these files later on). But if you're concerned about quality, you might want to replace the earbuds with something a little better.