The first 44100hz sample you posted is very good for testing because it has a wide range of frequencies: bass, middle and treble.
Format-wise, Opus maintains more the character of the original audio. AAC's bass sounds a bit more square while the treble is a bit higher, with a bit more noise in the background. Also we have to consider that most videos are uploaded to YouTube in AAC format, so I suppose it's easier to re-convert AAC to AAC than AAC to Opus.
Bitrate-wise, however, the 256kb AAC still has much more details and a richer sound than the Opus. But the details are something that most people probably don't notice. I find VP9 and Opus to keep more of the details that people notice, and to drop the details and noise.
That's testing with professional headphones that have a high accuracy reproduction of the whole audio spectrum, and a pre-amp plugged via USB that reduces the analog background noise.
I uploaded the 256kb AAC and 160kb Opus so that anyone can compare easily (the Opus is hard to grab from YouTube!)
https://mega.co.nz/#!XRA2TIKL!zvJNrFgVM … Pw_qpK0COw
https://mega.co.nz/#!nVIWwKZS!UPJa0MVug … 93rTdOinl0
EDIT: Wait! I was doing the tests with VLC. If I play in Windows Media Player, however, Opus sounds better! VLC's Opus decoder is bad.
In WMP or MPC-HC, I would say this file plays better in 128kb Opus.
Here are more audios to compare
32hz. Opus clearly sounds much better
https://mega.co.nz/#!nI5SVCBK!8U6c0hoK1 … j9X3kLoNJQ
https://mega.co.nz/#!6AhzGLQZ!PKV4JJgPR … s3Ol3yz82w
Real-life scenario. Inna - More Than Friends. It's a close call, but AAC sounds richer.
https://mega.co.nz/#!fJABmRSB!gogKytyR0 … ReVDWvavaE
https://mega.co.nz/#!rQJQXaiT!XoyhOwLUj … 4-uFY-rd50
Real-life scenario. Michel Teló - Ai Se Eu Te Pego
https://mega.co.nz/#!2IBWlBYT!wSvNlT3nZ … IODPygjops
https://mega.co.nz/#!KR5mFJ7C!XVyT2llIP … fT5bFwnKQw
The first 2 audios are pure electronic sounds with no background details so perhaps those are better encoded with Opus. For live recordings, however, it's more difficult to calculate the shape (Opus) and storing the pixelated details (AAC) still gives a richer sound.
For Michel Telo, the very beginning with simple audio sounds considerably better in Opus, and the rest of the music with complex sounds sounds better with AAC.
Opus works best for simple sounds or electronic sounds. AAC works best for complex sounds.
So I'll stick with AAC for now.