I would pick AA hands down. I've only really started flying DL lately and my experiences with them had been second to none. If my decision was mainly based on the hard product, I'd pick AA. Delta One is good but I still find AA Flagship Business a better hard product. I also find that AA's IFE is so much better with more selection than Delta. If you're flying from JFK, flying with AA will give you the chance to experience the new lounges. If I'm not mistaken, AA Flagship Business Plus passengers will be directed to the BA/AA Soho lounge (anyone please correct me if I'm wrong). Otherwise it will be the Greenwich lounge which is great in its own right too. If flying with Delta from JFK, good luck getting into the Sky Clubs on Concourse A or B. I was there recently and while I highly regard the Sky Club at JFK T4 Concourse B, you could find yourself waiting in line for up to 30 mins and they can get very crowded especially during peak times. You can also access the Virgin Clubhouse on Concourse A but the AA lounges are far more superior.

​​​​​When it comes to crew, AA is a hit and miss. I find their JFK crew to be just as good as their DFW and CLT based crew, warm and friendly compared to those BOS and MIA based. Delta FA in general are some of the friendliest I've come across with. I recently got back from a trip (FLL-LHR via JFK) and crew on both flights had been fantastic especially for a night flight on the JFK-LHR leg.

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