I'd go through AMS DL to KL. Then again, I know Schiphol pretty well.

AF from Atlanta in Y is extra miserable, with 10 wide seating on their 777s. The 747 flight may be a little better if they are still flying it. I'd take a DL 33L or 76W any day in Y. KL's 332 isn't bad, but the IFE needs updating. I haven't flown KL's 772. In Europe KL is standard Euro Y, no complaints, and I even kinda like flying City Hopper.

I've booked flights on DL metal via DL, KL metal through DL, DL metal through KL, and even KL metal through KL, and always had the ticket issued on 006 stock. This may be a function of my corporate travel agency though.