How difficult is Fusion/Quantum compared to PUN2?

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In my project I'm using PUN2 and it's not super difficult to work in it. Mostly I just check isMine, or RPCs or PhotonInstantiate.

Recently I found out about Fusion/Quantum and I was wondering is switching to the faster solutions a good idea? How much of an improvement are they from PUN2 and how much more difficult are they?

I should note that the game I'm working on is 4 player open world rpg and I've been doing good with PUN2 but now that I have come to AI I'm having difficulties and are the faster solutions better for AI?

Also I don't know if I should use Fusion or Quantum? Which is better for my case? Thanks in advance for any help.