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.