He's the safe bet for sure. I prefer the higher ceiling and lower floor choices. Because even if they flame out in 4 years, we're still in the same boat since McCarthy is likely to only last 4 years as well.
I get why some would rather go with the risk young coach. But the difference is that McCarthy, because he has proven recent and long term success as an NFL HC, is more likely to succeed than flame out which means in 4 years you have a better roster to sell to the next HC and likely to the next GM.
And I'm not sure why the younger guys would have a higher ceiling. I mean, the ceiling with McCarthy is established as winning a Super Bowl. If you want a higher ceiling, that's your choice, but I'm not sure you can know if any coach has a ceiling that would include winning more than one Super Bowl.
To me, McCarthy has both a high floor and a high ceiling. I wouldn't mind going with a younger OC like Brady, Moore, or Kingsbury, but I also think that McCarthy would be a good hire that would be hard to argue with.