If you care about status, DL operated flights will earn more MQMs if booked as DL-coded flights during 2021 due to Status Accelerator promotion. MQD and Mileage earnings may be more or less with Accelerator promo as AF booked flights with AF flight numbers will earn MQD/miles based on mileage-flown rather than fare paid. If you don't have DL status and don't care about earning it, just go with the AF booking and enter your Delta Skymiles number on AF website as noted above.