I personally think Potter's a good coach and that Poch's failures have vindicated him somewhat by showing the sheer difficulty of the task at Chelsea. But with that said there might be a cautionary tail there somewhere.
The task at United is not too dissimilar to the one at Chelsea. An underperforming club with a history of success which puts a lot of pressure and expectations on the coach, some huge transfer expenditure but still a lot of mediocre players in the squad, probably a bunch of strong personalities in the dressing room which the manager will need to win over/motivate etc. If he failed at Chelsea that doesn't make him a bad coach at all, but it might make him a coach not suitable for this particular task.
After seeing ETH stand meekly on the touchline and same with Rangnick before him, I think a more fiery coach might be needed to motivate and instil some confidence in the players. Which is why I'm leaning more towards De Zerbi as a better candidate. I know that Mediterranean fiery temperament is a double edged sword, but it's also worth remembering that Mourinho (another combustible character), for all his short-comings and career trajectory when he took over, he was still the one who managed to get the most out of this club in the 10 years post Fergie. And that Pep and Klopp, who are both very loud, opinionated and animated characters are doing very fine. Obviously I'm not trying to insinuate at all that:
loud, animated coach = good coach
I'm simply saying that it seems more of an advantage than a handicap, especially when dealing with star players. And aside from that particular character attribute, De Zerbi has also shown very good signs of being a progressive, front-foot football playing coach. So he would get my vote.