We have had two long threads comparing Sapphire Reserve with Platinum:
2016:
How will Amex counter the Chase Sapphire Reserve Card?
2017:
AMEX Platinum vs Chase Sapphire Reserve
I'm not sure there is much new to say on the topic, until the next refresh of either product.
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My own view is that Sapphire Reserve aligns more closely with Citi Prestige, and (as long as the annual fees are reduced by travel credits) I think there is a good case for holding Platinum plus one of the other two, but generally not all three.
FWIW, I think this thread really has a different focus that deserves a separate thread, so I hope we keep it. I've seen some of the comparison discussions, but it's interesting hearing from people who continue to maintain and hold both cards--they've already offered some insights I hadn't thought of.