Would you improve if the content required it?

[Zojha]

Simply put, the ability to adapt is far more difficult a challenge than the ability to memorize.

Where's the big difference between the two?

Either way, you're going to memorize what the boss can do and how to respond. Tank buster -> Focus heal on the tank. Spread mechanic -> Spread etc. These are pretty much the solutions to a given mechanic - you won't solve a stack mechanic by spreading or focus healing the tank, you'll just die. The only thing you really change is that instead of memorizing when things happen, you have a reaction time check. And depending on how good your reaction time happens to be vs how good you are at memorizing, that can be either more or less difficult.