iTunes encoders: AAC vs. mp3? and CBR vs. VBR?

Hello. Im importing various mp3's varying anywhere from 128kbps to 320kbps (majority of them closer to 320) and I want to convert them all using iTunes 9 to the same format. One that will even out the qualities of every song so that all of them are on the same level of sound quality when listening (and not just that they now all have the same number like 256VBR). I know about Apple lostless but those files are too large. So I'm only interested in the facts comparing AAC/mp3 and CBR/VBR. (I apologize for straying Im just trying to be specific about what I'm trying to accomplish). So my question is in order to accomplish this should I convert all to ACC or mp3(with all files one bitrate)? which bitrate ex. 128, 192, 256 ect.? and should I use a constant bit rate or variable bitrate?

If someone could help make suggestions and also explain a bit why it would be better for what Im trying to achieve it will be greatly appreciated. Thank you!
-Scott