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?
As others mentioned having a good encoder is an issue for AAC. And some skills in using it, tuning, etc.
Nearly all quality releasers now use AC3/EAC3 or FLAC. Tigole is the last one who uses AAC to my knowledge and the rest of the QXR group rolls their eyes at it.
You’re not going to get a meaningful reduction in bitrate and file size with AAC over EAC3/AC3 without loss of quality. We’re talking maybe you can shave 2-300kbps off an AAC version versus an AC3 5.1 track. And it’s tricky. So much so no one other than that one person I mentioned bothers. At least no one accepted in the higher echelons as competent in creating acceptably transparent encodes.
If a source has EAC3 (itself capable of up to halving the bitrate required vs AC3) or AC3 I’d recommend keeping it as they tend to already be efficient. They’re also universally compatible as codecs. Re-encode those big 1500kbps DTS tracks and those even bigger monster lossless Dolby and DTS tracks but I’d leave efficient codecs like AC3 alone.
That said it’s up to you what sounds good. If you’re using lower end stuff and can’t tell the difference after trying a few different test videos with different types of sounds then go for it.