Generally, I would charge to the Plat. 5x MR is going to be the best reward here.

The CSR benefits (delay protection) are much nicer in terms of administration; making a claim for the Amex insurance is a huge pain in terms of paperwork.

If "expensive" means premium cabin, I would generally recommend booking an IAP fare, although there is significant hassle dealing with Expedia dba Amex in that case - the savings can be fairly decent though.

Charging flights to a Delta branded card is basically a joke