It's too easy to jump on Stones because of him being exposed to much higher level of competition and also a lot of media attention (and the highlighting of errors which get shown over and over again and make people feel more confident taking extreme positions on a player) thanks to his high profile.
He's definitely a quality talent, maybe not worth his pricetag, but some people are being a bit too dismissive.
That said, there might be some definite flaws in his decision making/defensive positioning which could either just be a permanent trait of the player or be improved over time like with, say, Piqué.
I don't think it's ridiculous to suggest that Holding could end up being better than Stones, though, Holding might just prove to be the better defender. Still, I can appreciate @SRCJJ taking a cautious stance here, considering we really still haven't seen Holding against much good competition (Liverpool and that's it), nor over an extended period of time.
But the signs are certainly promising and him ending up better than Stones is certainly not out of the question.