Originally Posted by kiam

If you strictly choose only between CO and DL, I will choose DL. DL-AMEX Platinum credit card is GREAT with extra 10K MQMs miles each year after spending 25K and 10-12.5K miles after first purchase ona new account plus 5K MQMs mile upfront. If you plan to achieve 75K MQMs miles to get a Platinum Status (or already is one), then, "only in my opinion," DL is a way to go because you will get CRC Membership and 6 Upgrade Coupons (although I know you fly mostly on C and lounge access is not an issue to you, but upgrade coupons can become quite handy some times for you or your travelling companions) when you pay only Y, B or M (Economy Full Fare).

Hope this is helpful somehow.