Construction of the High Speed 2 rail line would mean £32 billion-worth of work, while adding a third runway at Heathrow would generate an additional £9bn.
But whether the industry gets to build either, neither, or both, turns not on any technical question but on raw politics.
Specifically, as last week’s further delay to the government’s consultation on South-east airports showed, it turns on an unresolved row within the Conservative party.
Last week, in an interview with Construction News, Olympic Delivery Authority chairman Sir John Armitt criticised the lack of ambition around high-speed rail as “not defendable”, saying there is “something wrong with the UK approach”.
He said: “The political side is fundamental. Today you still have people asking is HS2 on, or is it off.
“Ministers say it’s on but unfortunately politicians in the UK have so often flipped and flopped around these decisions that it is easy for people to feel, ‘they’ll probably change their mind if I push hard enough’.”
He added that the political debate around a third runway at Heathrow had shifted from environmental concerns to the impact of a lack of aviation capacity on the economy.
“[Two years ago] it was ‘we’re absolutely not going to have a third runway’; now two years later, ‘perhaps we may need a third runway’ because of economic and commercial pressures.
“What is it about us that we can’t put these two things together and debate them sensibly at the same time? It’s an incredibly immature approach we seem to take in this country where we can’t bring these things into one debate, get a political consensus and then take them forward.”
Although the government is committed to HS2 and against the third runway, huge lobbying efforts seek to change its mind.
The two issues are linked, as HS2 could feed huge numbers of passengers into Heathrow, driving the need for extra flight capacity.
An improbable combination of the business body London First and the GMB trade union is promoting the third runway on economic and employment grounds, and their campaign has the ear of some leading politicians.
Meanwhile, campaigners along the HS2 route argue it is questionable value for money, a pitch they hope will lead to its abandonment by a cash-strapped government.
The industry may not find out what it can build until after the 2015 general election. Labour, having once favoured both, now supports HS2 but opposes the third runway, a position shared by the Liberal Democrats.
When prime minister David Cameron sought to burnish the Tories’ ‘green’ credentials in 2010 he too took those same positions, which are enshrined in the coalition agreement.
But the Tories’ business supporters want both a third runway and HS2, and the Tory backbench Free Enterprise Group has even called for four runways.
The campaign against HS2 is led by councils along the route, almost all Tory-controlled, while London’s Conservative mayor wants Heathrow replaced by a new airport on an artificial island in the Thames, a plan strongly opposed by Tories in Kent.
If the industry wants to build the runway and HS2, it will need alliances with those battling for both. Independent transport consultant Mark Prior says the rows over the projects are “a clear case of politics interfering in infrastructure”.
“If politics were not involved, HS2 would not be built and the third runway would have been built years ago. The infrastructure investment in the third runway is quicker and easier,” he argues.
Mr Prior takes a bleak view of the prospects for any early start on construction. “I think they will have difficulty getting a hybrid bill for HS2 through Parliament, which the government will blame on objectors, and I doubt a third runway can be delivered this parliament,” he says.
“There is a case for investment in rail. The question is whether it needs to be HS2.”
He also questions how much benefit the UK industry would derive from HS2: “There would likely be a lot of work going to overseas consultants and contractors, as I do not think we have the expertise here.”
Although some HS2 supporters see it as substituting for domestic flights, and so reducing demand at Heathrow, Jim Steer, a director of consulting engineer Steer Davies Gleave, who also runs the pro-HS2 group Greengauge 21, says the two schemes are complementary.
Heathrow is the only UK ‘hub’ airport - meaning that unlike, say, Stansted or Manchester, it takes feeder traffic from domestic airports for intercontinental flights - and its lack of capacity has scared the airline industry, which sees passengers instead flying from UK regions to Paris or Amsterdam for connections.
Mr Steer explains: “High-speed rail can replace domestic flights to provide the ‘spokes’ to the hub. It would connect most of England with Heathrow - far more places than have flight links there. HS2 also creates a major surface transport hub at Heathrow.”
The planned Heathrow spur of HS2 is part of its second phase “but since it is relatively small it could be built earlier”, he says.
SKM Colin Buchanan technical director for economics John Sirruat agrees both could be built.
“They serve very different markets and I do not see them as competing,” he says. “If HS2 provides spokes for Heathrow you would have to expand the airport too, as people arriving on HS2 have to be able to fly somewhere.”
On the other side, Chris Castles, a transport economist who has studied HS2 for councils in its path, says: “The fundamental problem is that the basic economic case for HS2 is wrong.
“You could manage the [West Coast Main Line] capacity problem by lengthening trains and making £1bn of relatively modest improvements to the infrastructure. HS2 is a grossly expensive answer to the problem.”
At the root of disputes over Heathrow are the airlines, who are deeply frustrated by what they see as political interference trumping the economic case for the third runway.
Board of Airline Representatives UK chief executive Mike Carrivick says: “It is absolutely necessary that Heathrow has a third runway.
“If you were just looking at the total of runways around London there wouldn’t be a problem, but they are in the wrong places. They are not where the airlines want to be because Heathrow is a hub.”
Building a third runway would be politically brave for a government committed against it, and changing the Tories’ minds may be difficult given they have dozens of marginal seats near the airport, including that of transport secretary Justine Greening.
The lobbying campaign against HS2 is probably less influential than that in favour of the third runway. But the project’s sheer cost and lengthy timescale makes it a tempting target for savings, and at best only a long-term solution for better access to Heathrow.
To make sure the work flows, construction must work with the air and rail industries and marshal enough clout to stiffen the resolve of politicians faced with various well-funded and influential environmental objectors to both.
Who wins is likely to come down to who shouts loudest.
Alternatives to Heathrow for expansion
Gatwick - a planning restriction prevents a new runway before 2019, and would be a hub with few ‘spokes’;
Stansted - second runway plan sparked vociferous opposition, lacks ‘spokes’ and intercontinental flights;
Luton - can expand without a new runway, but otherwise same problems as Stansted;
Birmingham -plenty of spare capacity, but too far from London;
Manston - huge ex-military airport, but in remote Thanet location with poor transport links;
Lydd - awaiting outcome of planning application to expand but even more remote than Manston;
Northolt - will have capacity once RAF leaves but would need a high- speed rail link to nearby Heathrow.