I've found that it's less about the pad, and more about the compression/panels when it comes to centuries.

I prefer a bit more compression and a smoother leg gripper/cuff for longer rides. shorter rides don't matter as much.

I'm cheap and only have three pairs of bibs... my cannondale domestiques are good for around 50-60 miles... after that I go to my performance ultras... the performance elite are good too, but I like the strap structure less on longer rides.