In my experience the Centurion Lounges are like being in the main terminal but with better food and drink options. The lounges are universally overcrowded -- it is often hard to find a place to sit, so you don't get the sense of being somewhere more quiet than the terminal. That is rare in an Admiral's Club -- they are not generally overcrowded, but the food (if any) is considerably more basic.

I cancelled my Amex Platinum card after a few experiences in the Centurion Lounges (there weren't many other benefits of the card that I cared about), and use the Admiral's Club when I can, either because I'm in business class or flying internationally.