I have never flown, nor do I intend to go up in an aeroplane in the future. I have collected/dropped off people at Gatwick, Heathrow, Stansted and Luton airports.
It is essential that whichever of the two airports chosen for expansion has improved public transport links.
Heathrow is connected to London via the dire Piccadilly Underground line and the line from Paddington. It was planned originally for Heathrow to be on the HS2 route from London to the north, then it was to be served by a spur line, now it has been dropped. The lack of direct rail connections with the north, south and west puts pressure on the road system.
Gatwick is worse than Heathrow for rail connections. Apart from the overcrowded London-Brighton main line there is nothing apart from the slow service to Reading. Quite how the addition numbers using Gatwick will be served by public transport is a mystery.