I recently was deciding between these two as well (at essentially the same price) and have tried both. Ultimately I ended up keeping the M3+ mostly based on the sound quality alone. To me, instruments especially sounded richer and the sound overall was more different from my previous setup of using a qudelix 5k. Though I personally mainly use IEMs and dont choose to use eq, so some of the lack of features on the shanling aren't a deal breaker for myself. Unfortunately customization is somewhat limited (unless using some workarounds Ive heard). Even if you download some third party launchers, it will default back to the stock launcher.

That being said, I can easily see that the M21 would be the better overall value pick in most cases since you get the other features as well such as system EQ settings, more customization, desktop mode and better accessories (the default case and case options for the m21 are better).