If you plan on putting the songs in any mp3 players or anything you should use CBR (constant bitrate) for best compatibility. CBR is chosen alot of times because of its predictable file sizes but it will encode at a constant rate, say 128kbps wether you need it or not. It's pretty ineffecient compared to VBR (variable bitrate) encoding. VBR is the way to go for high quality if space isn't an issue. The only problem is unpredictable file sizes. It will constantly change the bit rate which is needed for the song. This means one 3 minute song may be 5mb's while a different 3 minute song could be 12 mb's. ABR (average bitrate) trys to take the best of both worlds. The user selects a bit rate (say 128kbps) and it will only change a few kbps (say up to ~180kbps) to help keep the file size in check.
oh, its a nice day. TO EAT CHILDREN!!!