Select the cheapest
My experience in transatlantic C class includes flights on CO, AF, KL, NW/DL, SK and LH. Some have been good, some have been so-so. And you can't seem to predict in advance which ones will be which. Therefore, I just go with the most economical choice -- which can amount to quite a bit of savings as I have seen RT fares ranging from USD 2200 to 7000.