I recently returned from a trip to BUD and used DL-AF for the whole trip because I was using a DL TCV. I could have flown NW-KL for about the same base fare but wanted to cash in the TCV as quickly as possible. All comments are based on limited AF experience.

I flew DL ONT-ATL-CDG over and ATL-ONT on the return. I flew AF for CDG-BUD-CDG-ATL. So far everything has posted to my NW account except for the CDG-BUD-CDG on AF which have posted to my DL account even though the boarding pass has my NW WP number; I've just submitted a request that this be fixed.

All of the posted flights earned 100% EQM. DL metal flights earned 100% plat bonus, the AF metal (even though DL coded) earned 125% plat bonus.

In the past I've flown international itineraries on NW-KL and CO-KL for comparison.

Some thoughts:
AMS is a much, much easier airport to connect through than CDG.

On the most recent trip, we were late in to CDG because of late departure from ATL. Parked on a hard stand but AF met the plane with a van for those of us connecting to BUD to assure we made our connections. On the return we were 30 minutes late arriving at CDG because of late arriving airplane at BUD. This was a 45 minute connection and although we parked at the terminal, there was no effort to get the 4 connecting passengers around two terminals to connect with the ATL flight. The plane was still connected to the jetway when I got there but the flight had closed. For me it was great because AF provided a night at their expense at CDG, lots of money for meals, and I got to spend the morning in Paris. And when I flashed my WP Platinum card, I got a better hotel. (If I'd needed to be home, though, I might have appreciated a connection that worked instead of the night at CDG.)

MSP and DTW (and even MEM) are much nicer to connect through than ATL (at least I think so).

The cross-the-pond food on AF was significantly better than anything I've been served in coach on either NW, CO, or DL.

The toilets on the plane were cleaner on AF than anything I've experienced recently on NW, CO, or DL. I got to talking with one of the AF flight attendants about it and she indicated that they are required to check the lavs every 15 minutes for fire safety, and to clean up a bit when they do. Don't know whether US crews have the same requirements.

KL and AF intra-Europe flights aren't very different -- except that the AF gate agents are much stricter about carry-ons that the KL gate agents.

The AF ST elite lounge at the terminal from which the trans-Atlantic flights depart was jam-packed. The KL lounges in AMS are full, but you couldn't move in the AF lounge. The DL lounges at ATL are nice, but not too different from what you get in MSP, DTW, MEM, IAH, or EWR.

So, for me I think I'd try to fly through AMS and use NW/KL unless you can get a connection using NW/AF that allows plenty of connection time at CDG. Full EQM, full bonuses, domestic upgrades.
tom