Just watching the Sky News debate now. In my opinion it has to be Heathrow RWY 3. AMS, DXB, FRA and even bloody CDG have been gutting LHR, and therefore the whole UK economy for years. I and several people I know actively avoid LHR because of the fragility of the schedule due to it being at capacity all day every day, when it used to be our default long haul transfer hub. Gatwick RWY #2 will not solve this problem. You'll get far more marginal routes to South America/China etc in LHR than you would in LGW.
The idea of extending the north runway to 6,000m and making it 2 runways works.,...assuming there's never a go around there again. Madness
I'm not a U.K. Tax payer, so it all depends on whether you folks paying the bills are willing to pay more to get more. In my opinion the tax payer contribution will be repaid by direct taxation at LHR and increased economic activity hundreds of times over in the long run.
Give Heathrow the go ahead, BUT if Gatwick still want to privately fund their own runway too, absolutely let them, as 10 years after this runway being debated now is completed, another will be needed.