yip, in general, opus offers a better audio quality than aac at the same bitrates, this is known and was discussed eg. at use lower bitrate for aac voice-messages · Issue #481 · deltachat/deltachat-android · GitHub
however, nevertheless, for now, aac is the better option for delta chat as it is supported natively on all platforms we’re targeting. this is not true for opus which would result in larger maintenance efforts then.
in the future, however, of course, we’d like to optimize that - maybe even going beyond opus then, eg. Codec 2 700C – Rowetel 