For me JFK both ways, no question.
VS3 is usually the ideal for me on the outbound, leaves LHR early doors and you are in Manhattan in time for lunch

Immigration is not bad, whether coming from the pointy end or if stuck behind the curtain, the day shift has more than double the amount of CBP desks open than the later arrivals so people are processed quickly in my experience.
VS46 on the return leaving from JFK would be my favourite, although I am amiable to the VS4 as well.... Customer service has always been great at JFK to me and despite my occasional disagreements with the TSA or self entitled lunatics that roam the halls, Clubhouse and staff are both top notch.
With the easy transfers into and from the city, JFK gets my vote y)