Originally Posted by
Weekend Away
Book the cheaper option. Once booked and ticket issued, take the ticket number (should be on your receipt) and look up your reservation on Delta.com using the ticket number. You'll be able to select you seats then. You can also manage the entire reservation there. Enter your FFN, upgrades, ect. Thankfully AF/KL and DL play nicely because of the transatlantic joint venture.
You can use AF/KL PNR on Delta web/app. It'll pull up the DL PNR automatically.