I expect the government will make the wrong decision but from a practical business point of view it has to be expand LHR. London needs a global class hub airport. Thats always been Heathrow ,
Giving LGW a second runway just creates two substandard hubs - you cannot provide the interchange needed to support a wide diversity fo destinations by splitting the traffic into two airports . expanding LGW is just throwing money away as it wont grow much and LHR will start to shrink as it cannot compete with AMS CDG and FRA .