streaming aac Vs. regular ole bitrates??

[G-daddy]

Can someone simplify aac for me and tell me why it's better than say 128kbps.
http://di.fm offers FREE aac yet charges for bitrates over 128 (generalization) yet aac sounds far superior to 96kbps so i would imagine that it's equiv. to 256 maybe? or better? i have no idea.

would the bandwidth be less while streaming aac?

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