Getting to LCY? Getting from AMS? Flight time + check-in time. 6 of this half a dozen of the other and a different experience for the kids. Chunnel.
LCY appears equidistant to the Andaz for example from London S Pancras by cab.
AMS is 20 minutes vs 8 Amsterdam CS from the Andaz there.
So only 12 minutes saved there.
BA says you can arrive at LCY as little as 20 minutes before departure, but even allowing for an hour at the airport vs 20 at the train, you've only saved 52 minutes outside of the actual train vs flight time.