Both are commercial enterprises, privately run and all future projects and construction should be privately funded. Government choosing one private company over another, involving public money is disgraceful. Approve both and allow them to fight it out
Except the UK is almost unique in having quite so many major parts of "public" infrastructure wholly in private hands. Private probation services, prisons, rail network and airports. Well Railtrack went bust and privatised rail is a monopoly licence to print money at taxpayers expense. The UKs only major hub airport, our main gateway to the world isn't just any other private enterprise and interacts with a whole load of public sector assets. It's quite clearly not something that you'd rationally expect to expand in that part of the world and be cost neutral to the taxpayer. Remember who built all those airports in the first place?
If they approve LHR R3, the ROI for LGW R2 collapses overnight alas.