Booking early is pretty key to get the better fares for this sort of thing if you want to travel at popular times, ie out Thursday/Friday morning, back Sunday afternoon for a city break.
If I were based in central London I'd much prefer to jump on a train there that would whisk me to straight into central Amsterdam than have to go through the hassles of two airports, as well as getting to the first airport and away from the second.
Also, as Gwylan says, being on a train, in...