bitrate
 

which is better, a higher or lower bitrate?

Higher = better quality. Better sound.

Higher!! That is if your after quality. The higher the bitrate the better the quality, but also the larger the file. If space is a concern then you may want to go with a lower bitrate. A bit rate of 128 is generally taken to be the low end of what's acceptable. Anything higher that that is better. 360 is about the highest you'll see in lossy format. Lossless files have bitrates over 1000. I'm no expert so search the forums for bitrate. It's been covered before ;).