What are the fare classes you're looking at?
If you're crediting to DL then CO R class doesn't get 150% mileage like other Business Class fares. At $3k I'm 99% sure its an R fare for CO and Z for NW and AF. NW & AF Z fares will both get you 150% mileage credit.
I'd lean pretty strongly to AF or NW. The CO seats aren't bad but the NW seats are a lot more confortable for sleeping.