Usually AAC is better than MP3, because it is the more modern codec that doesn't have some of the design flaws that MP3 has.
For most Pop / simple Rock stuff most people can't hear any differences at 128kps (both AAC and actual Lame 3.98) compared to the CD original.
With very critical music Lame -V0 is to be prefered to Lame -b 320, as the encoder is then using the bitrate reservoir in very critical passages (and could exceed 320 kpbs when needed) and dropping the bitrate when it isn't needed to be that high. AAC and Lame 3.98 have differnt types of problem sounds. While AAC has trouble with some types of electronical music (noise with lots of spartial effects), MP3 has often problems with dense accoustical stuff.
But for the stuff most of you guys are listening MP3 and AAC at 128kbps is enough. If you are into Classical, Folk, Metal, Noise, Industrial, Schranz, ... then better use the highest VBR setting, as you otherwise might hear the encoder flaws every now and then. If you are a paranoid audiophile then your only choice is a lossless codec like FLAC, WavPack or Apple Lossless.