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?
I'd honestly suggest using your ears and deciding if you can tell the difference. My music library is under 20GB so I do not fuss too much, but I am not truly sure I can tell the difference.