Just ask DL how much a M fare(the lowest fare you can upgrade on) is - usually double at least. And then see what the availability is. Take the U fare and tough it out. An extra night in a nice hotel is wonderful. Also you'll save a bunch of miles.

Also, I take the AF codeshares now as much as possible because they have free booze. Unless you are on a DL B777, take plenty to read or have your own entertainment available.