I always fly with Continental Airlines when I fly to the US as they have by far the best frequent flyer program. It is so quick and easy to earn free flights. Also I was able to fly my sister from SF to London for 25k miles last October. Can't beat that!
In terms of the flight, their planes are beautiful. The transatlantic flight is unsually on a new 777. The food isn't the best and the service is ok. If you prefer Virgin, you can fly on Virgin coshare flights and earn miles on Continental.