LGW may well be easier, for some but LHR will always retain the 'crown'. Note that I am not saying what the crown is for ...
I live these distances (in miles) from airports:
LTN = 11
LHR = 36
STN = 52
LGW = 75
So you can see why I do not wish to travel double the distance on the M25, or take a taxi/train/train to get to LGW.
As to where the congestion charging zone might start, the intention is for central London only, at a point inside of the North and South circular roads. How far inside - I do not know.
Since LHR is about 6.5 (approx. 10Km) from the Circular Road (different to the M25 circular motorway, for overseas readers!)