It almost certainly will be STN because that is politically the easy option.
However, from the ATC point of view it won't be easy, particularly on easterlies. It is 'easy' enough to get the extra airspace to the north for Westerly ops. but to the southwest there is too much in the way. City outbounds and inbounds, Luton outbounds through The Park and Heathrow/Northolt arrivals on top compressing it all to below 4000alt. That is 2 levels at best and in some areas 1. Vectoring into a cul-de-sac, with no room to orbit downwind, there would be nowhere to go in the event of trouble.
It would be like operating more traffic than Heathrow in half the airspace and bring in the same safety arguments. 4 runways at STN is going to take some major reorganisation of the TMA.
Point 4
