I find that i can hear a difference between Mp3Pro and Mp3, this is because Mp3Pro is encoded at 22Khz as opposed to Mp3's 44Khz. I used to use Mp3Pro all the time when i only had a small Mp3player,96kbps "cd transparency" however, is good quality audio, but does not sound better than a high quality Mp3.
VBR is useful for some songs, for example, if the song is 24 minutes long, where there is a song for 3 minutes, and then 18 minutes of silence and a bonus track for the last 3 minutes it will be useful as during the silence the bitrate will be low, this means the vbr file will take up less space, as opposed to the cbr file which would take up ~20mb at 128kbps.
WMA audio format is an aweful format invented and maintained only by microsoft (
) its compression is better than that of mp3, meaning a song encoded in 64kbps WMA will sound the same as the same song encoded in 128kbps MP3, the WMA file will also take up approx half the space of the MP3 file.