For me what's important is not the lounge as both LHR and LGW are very comparable in terms of setup and offer. Its more of flight timings and where are you going to / coming from in London. As mentioned above LGW provides easy access and quick to London Bridge/Victoria but if you are going to the west end then LHR makes more sense on the road or via the Piccadilly line. Add to that if you are in J/F and have chauffeur drive or just generally prefer a car to public transport then LHR is your best bet vs the long drive to LGW.
Agreed that this is another consideration.
Via public transport I would still opt for LGW, with the exception when you need to be near Paddington and are prepared to pay for the HEX. The Piccadilly line takes an hour to Central London.
Otherwise both London Bridge (for the City) and Victoria (which after all is also West end) are only 30 minutes by train from LGW.
Driving (which of course includes CD), I would agree that LHR would probably be a bit better. But then driving in London is not fun at the best of times although having a chauffeur makes it tolerable.
And although the lounges are comparable, LHR is still a zoo...