I would go with BA. I have flown Delta twice this year to the States, once on a 767-300 old style seats and once on a 757 (report pending). The service can be very good but the food not so. The seats are ok and I managed to sleep no problem. The pre clearance in DUB is not that brilliant. I was queuing for well over an hour and got called forward along with many others. The lounge in DUB is nothing to shout about either, very meagre offerings for breakfast i.e. no breakfast items apart from cereal! BA, you will get to use the fabulous lounge, fly on a 777 and generally receive a good, consistent service. I would go with Delta again if there was a big saving, in my case I went J for 1450,00 eur – a bargain, but if no difference in price, than BA.