In the past I’ve chosen I’ve often kept AC3 audio tracks thinking that their substantially higher bitrates made them better than the AAC tracks I compared them to. As I’ve since learned that AAC can be comparable to AC3 at a substantially lower bitrate, to have a means of comparing the two codecs, what would the AAC-equivalent bitrates be for 224kbps and 640kbps AC3?
From a quick look on wikipedia, looks like AC3 does not support VBR. That is enough to make AAC twice as good at least, especially since movies have a lot of silence in them, so your ratio of 1:2 equivalence seems right to me