[The Other Tom]

I haven't used the defender, but lots of people use CC2 and are happy with them. They do have some drawbacks, though (~10% lower range, loud) compared to the OEM self seals. I assume the defenders would have the same issues.
Then there's this:

Michelin Defender 2 Vs CrossClimate 2: Which Tire Wins? - Drive Cruise

When it comes to choosing the right tires for our vehicles, we're all looking for that perfect blend of durability, performance, and safety. Enter the debate

drivecruise.com

 

[The Other Tom]

However longer mileage warranty, Costco is not giving any prorated discount.

Not surprising.
The tire manufacturer honors the warranty, not Costco. Go to discount tire and see what they say. They deal with this sort of thing all the time.