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).

However, if you plan to fly CO/NW a lot within the next 12 months, then go for CO because CO and NW are 100% compatible to each other while DL still kind of not really completely migrates into CO and NW regulations and benefits as of yet (vice versa). Once all 3 are fully together, then I don't think it matters much which program you are in. CO credit cards suck, in my opinion, less bonus and no elite qualifying miles given.

If you plan to achieve MILLION miles, then CO or NW does not recognize Million Miler Status as of yet, DL does.

Hope this is helpful somehow.