Washington and NYC

Right, I am planning to go to Washington and NYC for a week at the end of the Aug/Sept. Went to NYC last summer and loved it so much that I am planning to go again this year. Will my love affair with NYC fizzled out second time around?
The plans are to fly VS LHR-IAD and return from JFK-LHR. How does the pricing work in this instance compare to LHR-IAD-LHR or LHR-JFK-LHR? Are there premiums to pay if you arrive and depart from different airports? I think VS did a sale in May/June last year so I might hold out until then as I made the mistake and booked early last year and paid an extra 80 for the tickets each.
Once in Washington what are there to do? I am planning to stay there for about 3 days and 4 days will be spent in NYC. Are 3 days long enough for Washington and are places easy to get to on foot or by public transport? What do I go to see in 3 days?
Furthermore travelling between Washington and New York, what are your suggestions? Planes, Trains or Automobiles? The trains take me to Penn Station but ofcourse flights gets me to the outskirts (possible DL miles earning).
The plans are to fly VS LHR-IAD and return from JFK-LHR. How does the pricing work in this instance compare to LHR-IAD-LHR or LHR-JFK-LHR? Are there premiums to pay if you arrive and depart from different airports? I think VS did a sale in May/June last year so I might hold out until then as I made the mistake and booked early last year and paid an extra 80 for the tickets each.
Once in Washington what are there to do? I am planning to stay there for about 3 days and 4 days will be spent in NYC. Are 3 days long enough for Washington and are places easy to get to on foot or by public transport? What do I go to see in 3 days?
Furthermore travelling between Washington and New York, what are your suggestions? Planes, Trains or Automobiles? The trains take me to Penn Station but ofcourse flights gets me to the outskirts (possible DL miles earning).