Generally speaking, the higher the bitrate, the better the sound quality. Higher bitrates will also give you larger files. You may not be able to distinguish between the mp3s you mentioned because the bitrates are similar (mp3s range from 96 to 320 kbps I think), because the song is fairly consistent (doesn't range from quiet to loud), or because you personally just can't tell the difference.

Here's a link that I found useful:

What is bitrate?

Hope this helps