I'd look through all the previous information here, but this is one case where YMMV greatly. You should review and determine what factors are most important to you.
My personal opinion is that I don't see much difference, and generally give to the edge to AF. However, I flew BE on DL this month JFK-LHR RT, and Mrs. TrojanTraveler flew separately. The FA service on all our flights was mediocre at best. I would give it a C or C-. I was very disappointed (especially given that DL positions this as a premier run to compete on, flat seats, etc.), and expected better. It's one thing to get bad service on one flight, but on all of them? I'm not expected east asian airline levels of service, but really...