My go-to airline is Virgin Atlantic. Never had anything but wonderful service with them (bar the shitness of Terminal 3 at Heathrow).
Going to New York in September and United are about £20 cheaper.
You reckon it's worth taking the risk that United are crap for the £20, since it's only like 8 hours on the plane? Or go with your trusted one for extra bunse?
Also - is it easier to get public transport to Manhattan (Upper West Side) from JFK or Newark? They look sort of equidistant.